2002-09-30 15:01:33

by Alexander Hoogerhuis

[permalink] [raw]
Subject: CPU/cache detection wrong

Accroding to my kernel, this is what i got:

CPU: Before vendor init, caps: 3febf9ff 00000000 00000000, vendor = 0
CPU: L1 I cache: 0K, L1 D cache: 8K
CPU: L2 cache: 512K
CPU: After vendor init, caps: 3febf9ff 00000000 00000000 00000000
Intel machine check architecture supported.
Intel machine check reporting enabled on CPU#0.
CPU: After generic, caps: 3febf9ff 00000000 00000000 00000000
CPU: Common caps: 3febf9ff 00000000 00000000 00000000
CPU: Intel(R) Pentium(R) 4 Mobile CPU 1.70GHz stepping 04
Enabling fast FPU save and restore... done.
Enabling unmasked SIMD FPU exception support... done.
Checking 'hlt' instruction... OK.

The machine is a Comapq Evo n800c with a 1.7GHz P4-M in it, and
according to the BIOS I've got 16kb/512Kb L1/L2-cache. Accroding to
the 2.4.20-pre7-ac3-kernel. It's been like this at least since
2.4.19-pre4 or so.

mvh,
A
--
Alexander Hoogerhuis | [email protected]
CCNP - CCDP - MCNE - CCSE | +47 908 21 485
"You have zero privacy anyway. Get over it." --Scott McNealy


2002-09-30 16:22:10

by Alan Cox

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

On Sat, 2002-09-28 at 13:29, Alexander Hoogerhuis wrote:
> CPU: Intel(R) Pentium(R) 4 Mobile CPU 1.70GHz stepping 04
> Enabling fast FPU save and restore... done.
> Enabling unmasked SIMD FPU exception support... done.
> Checking 'hlt' instruction... OK.
>
> The machine is a Comapq Evo n800c with a 1.7GHz P4-M in it, and
> according to the BIOS I've got 16kb/512Kb L1/L2-cache. Accroding to
> the 2.4.20-pre7-ac3-kernel. It's been like this at least since
> 2.4.19-pre4 or so.

Can you stick a printk in arch/i386/kernel/setup.c in the function
init_intel

Just before:
/* look up this descriptor in the table */

stick

printk("Cache info byte: %02X\n", des);

that will dump the cache info out of the CPU as the kernel scans it and
should let us find the error in the table.


2002-09-30 17:38:12

by Alexander Hoogerhuis

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

Alan Cox <[email protected]> writes:

> On Sat, 2002-09-28 at 13:29, Alexander Hoogerhuis wrote:
> > CPU: Intel(R) Pentium(R) 4 Mobile CPU 1.70GHz stepping 04
> > Enabling fast FPU save and restore... done.
> > Enabling unmasked SIMD FPU exception support... done.
> > Checking 'hlt' instruction... OK.
> >
> > The machine is a Comapq Evo n800c with a 1.7GHz P4-M in it, and
> > according to the BIOS I've got 16kb/512Kb L1/L2-cache. Accroding to
> > the 2.4.20-pre7-ac3-kernel. It's been like this at least since
> > 2.4.19-pre4 or so.
>
> Can you stick a printk in arch/i386/kernel/setup.c in the function
> init_intel
>
> Just before:
> /* look up this descriptor in the table */
>
> stick
>
> printk("Cache info byte: %02X\n", des);
>
> that will dump the cache info out of the CPU as the kernel scans it and
> should let us find the error in the table.
>

And the jury says:

PU: Before vendor init, caps: 3febf9ff 00000000 00000000, vendor = 0
Cache info byte: 50
Cache info byte: 5B
Cache info byte: 66
Cache info byte: 00
Cache info byte: 00
Cache info byte: 00
Cache info byte: 00
Cache info byte: 00
Cache info byte: 00
Cache info byte: 00
Cache info byte: 00
Cache info byte: 40
Cache info byte: 70
Cache info byte: 7B
Cache info byte: 00
CPU: L1 I cache: 0K, L1 D cache: 8K
CPU: L2 cache: 512K
CPU: After vendor init, caps: 3febf9ff 00000000 00000000 00000000

Let me know if you need more info :)

mvh,
A
--
Alexander Hoogerhuis | [email protected]
CCNP - CCDP - MCNE - CCSE | +47 908 21 485
"You have zero privacy anyway. Get over it." --Scott McNealy

2002-09-30 22:07:52

by Dave Jones

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

On Mon, Sep 30, 2002 at 07:43:16PM +0200, Alexander Hoogerhuis wrote:

> PU: Before vendor init, caps: 3febf9ff 00000000 00000000, vendor = 0
> Cache info byte: 50

Instruction TLB (ignored)

> Cache info byte: 5B

Data TLB (ignored)

> Cache info byte: 66

8K L1 data cache

> Cache info byte: 00
> Cache info byte: 00
> Cache info byte: 00
> Cache info byte: 00
> Cache info byte: 00
> Cache info byte: 00
> Cache info byte: 00
> Cache info byte: 00

Null

> Cache info byte: 40

No 3rd level cache.

> Cache info byte: 70

12K-uops trace cache

> Cache info byte: 7B

512K L2 cache

> Cache info byte: 00

Null.

> CPU: L1 I cache: 0K, L1 D cache: 8K
> CPU: L2 cache: 512K

So all is correct, except for the missing trace cache entries.

Patch below adds several missing descriptors, fixes up
an errata workaround, and adds reporting for the trace cache.
I've no Intel hardware to test this code with, so if people can
make sure I've not broken anything, that'd be good (I hate touching this code).

Alex, with this patch, you should see..

CPU: Trace cache: 12K uops, L1 D cache: 8K
CPU: L2 cache: 512K

Confirmed ?

Dave

diff -urpN --exclude-from=/home/davej/.exclude linux-2.4.20-pre8/arch/i386/kernel/setup.c linux-2.4.20-pre8-leakfix/arch/i386/kernel/setup.c
--- linux-2.4.20-pre8/arch/i386/kernel/setup.c 2002-09-26 15:29:12.000000000 +0100
+++ linux-2.4.20-pre8-leakfix/arch/i386/kernel/setup.c 2002-09-30 23:03:04.000000000 +0100
@@ -1256,15 +1256,6 @@ static void __init display_cacheinfo(str
l2size = 256;
}

- /* Intel PIII Tualatin. This comes in two flavours.
- * One has 256kb of cache, the other 512. We have no way
- * to determine which, so we use a boottime override
- * for the 512kb model, and assume 256 otherwise.
- */
- if ((c->x86_vendor == X86_VENDOR_INTEL) && (c->x86 == 6) &&
- (c->x86_model == 11) && (l2size == 0))
- l2size = 256;
-
/* VIA C3 CPUs (670-68F) need further shifting. */
if (c->x86_vendor == X86_VENDOR_CENTAUR && (c->x86 == 6) &&
((c->x86_model == 7) || (c->x86_model == 8))) {
@@ -2180,6 +2171,7 @@ extern void trap_init_f00f_bug(void);
#define LVL_1_DATA 2
#define LVL_2 3
#define LVL_3 4
+#define LVL_TRACE 5

struct _cache_table
{
@@ -2199,6 +2191,8 @@ static struct _cache_table cache_table[]
{ 0x23, LVL_3, 1024 },
{ 0x25, LVL_3, 2048 },
{ 0x29, LVL_3, 4096 },
+ { 0x39, LVL_2, 128 },
+ { 0x3C, LVL_2, 256 },
{ 0x41, LVL_2, 128 },
{ 0x42, LVL_2, 256 },
{ 0x43, LVL_2, 512 },
@@ -2207,11 +2201,15 @@ static struct _cache_table cache_table[]
{ 0x66, LVL_1_DATA, 8 },
{ 0x67, LVL_1_DATA, 16 },
{ 0x68, LVL_1_DATA, 32 },
+ { 0x70, LVL_TRACE, 12 },
+ { 0x71, LVL_TRACE, 16 },
+ { 0x72, LVL_TRACE, 32 },
{ 0x79, LVL_2, 128 },
{ 0x7A, LVL_2, 256 },
{ 0x7B, LVL_2, 512 },
{ 0x7C, LVL_2, 1024 },
{ 0x82, LVL_2, 256 },
+ { 0x83, LVL_2, 512 },
{ 0x84, LVL_2, 1024 },
{ 0x85, LVL_2, 2048 },
{ 0x00, 0, 0}
@@ -2219,7 +2217,7 @@ static struct _cache_table cache_table[]

static void __init init_intel(struct cpuinfo_x86 *c)
{
- unsigned int l1i = 0, l1d = 0, l2 = 0, l3 = 0; /* Cache sizes */
+ unsigned int trace = 0, l1i = 0, l1d = 0, l2 = 0, l3 = 0; /* Cache sizes */
char *p = NULL;
#ifndef CONFIG_X86_F00F_WORKS_OK
static int f00f_workaround_enabled = 0;
@@ -2279,8 +2277,10 @@ static void __init init_intel(struct cpu
case LVL_3:
l3 += cache_table[k].size;
break;
+ case LVL_TRACE:
+ trace += cache_table[k].size;
+ break;
}
-
break;
}

@@ -2288,9 +2288,25 @@ static void __init init_intel(struct cpu
}
}
}
- if ( l1i || l1d )
- printk(KERN_INFO "CPU: L1 I cache: %dK, L1 D cache: %dK\n",
- l1i, l1d);
+
+ /* Intel PIII Tualatin. This comes in two flavours.
+ * One has 256kb of cache, the other 512. We have no way
+ * to determine which, so we use a boottime override
+ * for the 512kb model, and assume 256 otherwise.
+ */
+ if ((c->x86 == 6) && (c->x86_model == 11) && (l2 == 0))
+ l2 = 256;
+ /* Allow user to override all this if necessary. */
+ if (cachesize_override != -1)
+ l2 = cachesize_override;
+
+ if ( trace )
+ printk (KERN_INFO "CPU: Trace cache: %dK uops", trace);
+ else if ( l1i )
+ printk (KERN_INFO "CPU: L1 I cache: %dK", l1i);
+ if ( l1d )
+ printk(", L1 D cache: %dK\n", l1d);
+
if ( l2 )
printk(KERN_INFO "CPU: L2 cache: %dK\n", l2);
if ( l3 )

--
| Dave Jones. http://www.codemonkey.org.uk

2002-10-01 01:00:52

by David L. DeGeorge

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

I too have incorrect CPU/cache detection using 2.4.20-pre7-ac3. I seem to
recall it working correctly on 2.4.19-ac1 (this was the version in which the
ac did not get added by the patch). Anyway I have a tualatin PIII with a 512K
L2 cache. I inserted the printk in setup.c that Alan asked for earlier in
the thread and got three outputs (it's a two processor machine). I have
edited everything but the printk output and the CPU:... lines. Things are
working fine with the kernel command line cachesize=512.
David
Sep 30 12:56:02 janus kernel: Cache info byte: 01
Sep 30 12:56:02 janus kernel: Cache info byte: 02
Sep 30 12:56:02 janus kernel: Cache info byte: 03
Sep 30 12:56:02 janus kernel: Cache info byte: 00
Sep 30 12:56:02 janus kernel: Cache info byte: 00
Sep 30 12:56:02 janus last message repeated 6 times
Sep 30 12:56:02 janus kernel: Cache info byte: 83
Sep 30 12:56:02 janus kernel: Cache info byte: 08
Sep 30 12:56:02 janus kernel: Cache info byte: 04
Sep 30 12:56:02 janus kernel: Cache info byte: 0C
Sep 30 12:56:02 janus kernel: CPU: L1 I cache: 16K, L1 D cache: 16K
Sep 30 12:56:02 janus kernel: CPU: L2 cache: 256K
...........................................................
Sep 30 12:56:02 janus kernel: Cache info byte: 01
Sep 30 12:56:02 janus kernel: Cache info byte: 02
Sep 30 12:56:02 janus kernel: Cache info byte: 03
Sep 30 12:56:02 janus kernel: Cache info byte: 00
Sep 30 12:56:02 janus last message repeated 7 times
Sep 30 12:56:02 janus kernel: Cache info byte: 83
Sep 30 12:56:02 janus kernel: Cache info byte: 08
Sep 30 12:56:02 janus kernel: Cache info byte: 04
Sep 30 12:56:02 janus kernel: Cache info byte: 0C
Sep 30 12:56:02 janus kernel: CPU: L1 I cache: 16K, L1 D cache: 16K
Sep 30 12:56:02 janus kernel: CPU: L2 cache: 256K
.....................................................................................
Sep 30 12:56:02 janus kernel: Cache info byte: 01
Sep 30 12:56:02 janus kernel: Cache info byte: 02
Sep 30 12:56:02 janus kernel: Cache info byte: 03
Sep 30 12:56:02 janus kernel: Cache info byte: 00
Sep 30 12:56:02 janus last message repeated 7 times
Sep 30 12:56:02 janus kernel: Cache info byte: 83
Sep 30 12:56:02 janus kernel: Cache info byte: 08
Sep 30 12:56:02 janus kernel: Cache info byte: 04
Sep 30 12:56:02 janus kernel: Cache info byte: 0C
Sep 30 12:56:02 janus kernel: CPU: L1 I cache: 16K, L1 D cache: 16K
Sep 30 12:56:02 janus kernel: CPU: L2 cache: 256K

2002-10-01 07:16:26

by Alexander Hoogerhuis

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

Dave Jones <[email protected]> writes:

> On Mon, Sep 30, 2002 at 07:43:16PM +0200, Alexander Hoogerhuis wrote:
>
> > PU: Before vendor init, caps: 3febf9ff 00000000 00000000, vendor = 0
> > Cache info byte: 50
>
> Instruction TLB (ignored)
>
> > Cache info byte: 5B
>
> Data TLB (ignored)
>
> > Cache info byte: 66
>
> 8K L1 data cache
>
> > Cache info byte: 00
> > Cache info byte: 00
> > Cache info byte: 00
> > Cache info byte: 00
> > Cache info byte: 00
> > Cache info byte: 00
> > Cache info byte: 00
> > Cache info byte: 00
>
> Null
>
> > Cache info byte: 40
>
> No 3rd level cache.
>
> > Cache info byte: 70
>
> 12K-uops trace cache
>
> > Cache info byte: 7B
>
> 512K L2 cache
>
> > Cache info byte: 00
>
> Null.
>
> > CPU: L1 I cache: 0K, L1 D cache: 8K
> > CPU: L2 cache: 512K
>

Here we go:

CPU: Trace cache: 12K uops, L1 D cache: 8K
CPU: L2 cache: 512K

But my BIOS still say I should have 8Kb/8Kb I/D L1 cache... oh
well. I'm sure Alan Cox would just write it up as marketing, since
thats about how reliable a BIOS is :)

ttfn,
A
--
Alexander Hoogerhuis | [email protected]
CCNP - CCDP - MCNE - CCSE | +47 908 21 485
"You have zero privacy anyway. Get over it." --Scott McNealy

2002-10-01 10:58:44

by Dave Jones

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

On Tue, Oct 01, 2002 at 09:21:26AM +0200, Alexander Hoogerhuis wrote:
> Dave Jones <[email protected]> writes:
>
> > On Mon, Sep 30, 2002 at 07:43:16PM +0200, Alexander Hoogerhuis wrote:
> >
> > > PU: Before vendor init, caps: 3febf9ff 00000000 00000000, vendor = 0
> > > Cache info byte: 50
> >
> > Instruction TLB (ignored)
> >
> > > Cache info byte: 5B
> >
> > Data TLB (ignored)
> >
> > > Cache info byte: 66
> >
> > 8K L1 data cache
> >
> > > Cache info byte: 00
> > > Cache info byte: 00
> > > Cache info byte: 00
> > > Cache info byte: 00
> > > Cache info byte: 00
> > > Cache info byte: 00
> > > Cache info byte: 00
> > > Cache info byte: 00
> >
> > Null
> >
> > > Cache info byte: 40
> >
> > No 3rd level cache.
> >
> > > Cache info byte: 70
> >
> > 12K-uops trace cache
> >
> > > Cache info byte: 7B
> >
> > 512K L2 cache
> >
> > > Cache info byte: 00
> >
> > Null.
> >
> > > CPU: L1 I cache: 0K, L1 D cache: 8K
> > > CPU: L2 cache: 512K
> >
>
> Here we go:
>
> CPU: Trace cache: 12K uops, L1 D cache: 8K
> CPU: L2 cache: 512K
>
> But my BIOS still say I should have 8Kb/8Kb I/D L1 cache... oh
> well. I'm sure Alan Cox would just write it up as marketing, since
> thats about how reliable a BIOS is :)

Hmm, can a P4 have a trace cache AND an L1 I cache ?
I thought they were exclusive, which is why the code
doesn't take this into account. Easily fixed if so though..

Dave

--
| Dave Jones. http://www.codemonkey.org.uk

2002-10-01 11:09:50

by Dave Jones

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

On Mon, Sep 30, 2002 at 09:06:10PM -0400, David L. DeGeorge wrote:
> I too have incorrect CPU/cache detection using 2.4.20-pre7-ac3. I seem to
> recall it working correctly on 2.4.19-ac1 (this was the version in which the
> ac did not get added by the patch). Anyway I have a tualatin PIII with a 512K
> L2 cache.

Some of the tualatins have an errata which makes L2 cache sizing
impossible. They actually report they have 0K L2 cache. By checking
the CPU model, we can guess we have at least 256K (which is where Linux
got that number from in your case). But this however means the 512K
models will report as 256K too.
To work around it, boot with cachesize=512 and all will be good.

Dave

--
| Dave Jones. http://www.codemonkey.org.uk

2002-10-01 13:30:09

by Maciej W. Rozycki

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

On Tue, 1 Oct 2002, Dave Jones wrote:

> Some of the tualatins have an errata which makes L2 cache sizing
> impossible. They actually report they have 0K L2 cache. By checking
> the CPU model, we can guess we have at least 256K (which is where Linux
> got that number from in your case). But this however means the 512K
> models will report as 256K too.
> To work around it, boot with cachesize=512 and all will be good.

Strange -- why not to default to 256K and override it with the value
obtained from a cache descriptor if != 0, then?

--
+ Maciej W. Rozycki, Technical University of Gdansk, Poland +
+--------------------------------------------------------------+
+ e-mail: [email protected], PGP key available +


2002-10-01 15:07:15

by Dave Jones

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

On Tue, Oct 01, 2002 at 03:35:54PM +0200, Maciej W. Rozycki wrote:


> > Some of the tualatins have an errata which makes L2 cache sizing
> > impossible. They actually report they have 0K L2 cache. By checking
> > the CPU model, we can guess we have at least 256K (which is where Linux
> > got that number from in your case). But this however means the 512K
> > models will report as 256K too.
> > To work around it, boot with cachesize=512 and all will be good.
>
> Strange -- why not to default to 256K and override it with the value
> obtained from a cache descriptor if != 0, then?

Because the cache descriptor IS zero. So we default to 256K.

Dave

--
| Dave Jones. http://www.codemonkey.org.uk

2002-10-01 15:26:04

by Alexander Hoogerhuis

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

Dave Jones <[email protected]> writes:

> On Tue, Oct 01, 2002 at 09:21:26AM +0200, Alexander Hoogerhuis wrote:
> > Here we go:
> >
> > CPU: Trace cache: 12K uops, L1 D cache: 8K
> > CPU: L2 cache: 512K
> >
> > But my BIOS still say I should have 8Kb/8Kb I/D L1 cache... oh
> > well. I'm sure Alan Cox would just write it up as marketing, since
> > thats about how reliable a BIOS is :)
>
> Hmm, can a P4 have a trace cache AND an L1 I cache ?
> I thought they were exclusive, which is why the code
> doesn't take this into account. Easily fixed if so though..
>

I don't know the gory details of it, but my BIOS claims I got 8/8, but
I'm deep enough in it now to start taking the 5th amendment on the
details here :)

ttfn,
A
--
Alexander Hoogerhuis | [email protected]
CCNP - CCDP - MCNE - CCSE | +47 908 21 485
"You have zero privacy anyway. Get over it." --Scott McNealy

2002-10-01 15:57:12

by Maciej W. Rozycki

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

On Tue, 1 Oct 2002, Dave Jones wrote:

> > > Some of the tualatins have an errata which makes L2 cache sizing
> > > impossible. They actually report they have 0K L2 cache. By checking
> > > the CPU model, we can guess we have at least 256K (which is where Linux
> > > got that number from in your case). But this however means the 512K
> > > models will report as 256K too.
> > > To work around it, boot with cachesize=512 and all will be good.
> >
> > Strange -- why not to default to 256K and override it with the value
> > obtained from a cache descriptor if != 0, then?
>
> Because the cache descriptor IS zero. So we default to 256K.

You wrote "some of", so I suppose others are OK. I meant those others.
Anyway -- is it a new problem? I can't see it documented in the current
P3 spec update. That's weird -- Intel might hesitate documenting
weirdnesses of their chips, however they tend to include such simple and
obvious errata in the update.

The spec actually states the L2 descriptor for the P3 may be as follows:

- 0x43 -- 512kB, unified,

- 0x82 -- 256kB, 8-way set associative,

- 0x83 -- 512kB, 8-way set associative.

The last descriptor is omitted from the list of known types in cache_table
in 2.4.20-pre8 -- could it be the culprit?

Maciej

--
+ Maciej W. Rozycki, Technical University of Gdansk, Poland +
+--------------------------------------------------------------+
+ e-mail: [email protected], PGP key available +

2002-10-01 16:23:17

by Dave Jones

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

On Tue, Oct 01, 2002 at 06:03:03PM +0200, Maciej W. Rozycki wrote:

> > > Strange -- why not to default to 256K and override it with the value
> > > obtained from a cache descriptor if != 0, then?
> > Because the cache descriptor IS zero. So we default to 256K.
> You wrote "some of", so I suppose others are OK.

Yes, one stepping only afaik. Though the test in the kernel checks
for 6.11,x with l2==0 just to be sure.

> I meant those others.
> Anyway -- is it a new problem? I can't see it documented in the current
> P3 spec update. That's weird -- Intel might hesitate documenting
> weirdnesses of their chips, however they tend to include such simple and
> obvious errata in the update.
>
> The spec actually states the L2 descriptor for the P3 may be as follows:
>
> - 0x43 -- 512kB, unified,
> - 0x82 -- 256kB, 8-way set associative,
> - 0x83 -- 512kB, 8-way set associative.
>
> The last descriptor is omitted from the list of known types in cache_table
> in 2.4.20-pre8 -- could it be the culprit?

Added in last nights patch. IIRC, the errata meant there was no
descriptor at all iirc.
TBH, I can't recall which document I read about this in, as it was a few
months back now. I'll look through old mails when I get chance and see if
I can get to the bottom of it.
It's possible that this was reported to me, and it is the case you
describe (missing descriptor), but the old code should have picked
apart the descriptors in a different way to the new table-lookup,
so that *should* have worked ok if the descriptor was correct.

Dave

--
| Dave Jones. http://www.codemonkey.org.uk

2002-10-01 16:56:54

by Vojtech Pavlik

[permalink] [raw]
Subject: Re: CPU/cache detection wrong

On Tue, Oct 01, 2002 at 05:31:07PM +0200, Alexander Hoogerhuis wrote:
> Dave Jones <[email protected]> writes:
>
> > On Tue, Oct 01, 2002 at 09:21:26AM +0200, Alexander Hoogerhuis wrote:
> > > Here we go:
> > >
> > > CPU: Trace cache: 12K uops, L1 D cache: 8K
> > > CPU: L2 cache: 512K
> > >
> > > But my BIOS still say I should have 8Kb/8Kb I/D L1 cache... oh
> > > well. I'm sure Alan Cox would just write it up as marketing, since
> > > thats about how reliable a BIOS is :)
> >
> > Hmm, can a P4 have a trace cache AND an L1 I cache ?
> > I thought they were exclusive, which is why the code
> > doesn't take this into account. Easily fixed if so though..
> >
>
> I don't know the gory details of it, but my BIOS claims I got 8/8, but
> I'm deep enough in it now to start taking the 5th amendment on the
> details here :)

12k of trace cache and 8k of I-cache are more or less equivalent - trace
cache is less effective because it has decoded uops, while i-cache has
encoded instructions, which take up less space.

--
Vojtech Pavlik
SuSE Labs