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Subject: Re: [PATCH 6/6] x86/vdso: Move out the CPU number store
To:     "H. Peter Anvin" <hpa@zytor.com>,
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Date:   Tue, 5 Jun 2018 09:03:48 +0300
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On 06/05/2018 08:36 AM, H. Peter Anvin wrote:
> On 06/04/18 20:57, Mika Penttilä wrote:
>>
>> This won't work on X86-32 because it actually uses the segment limit with fs: access. So there 
>> is a reason why the lsl based method is X86-64 only.
>>
> 
> <thinks out loud>
> 
> Why does that matter in any shape, way, or form?  The LSL instruction
> doesn't touch any of the segment registers, it just uses a segment
> selector number.
> 
> <looks at code>
> 
> I see... we have a VERY unfortunate name collision: the x86-64
> GDT_ENTRY_PERC_PU and the i386 GDT_ENTRY_PERCPU are in fact two
> completely different things, with the latter being the actual percpu
> offset used by the kernel.
> 
> So yes, this patch is wrong, because the naming of the x86-64 segment is
> insane especially in the proximity of the  -- it should be something
> like GDT_ENTRY_CPU_NUMBER.
> 
> Unfortunately we probably can't use the same GDT entry on x86-32 and
> x86-64, because entry 15 (selector 0x7b) is USER_DS, which is something
> we really don't want to screw with.  This means i386 programs that
> execute LSL directly for whatever reason will have to understand the
> difference, but most of the other segment numbers differ as well,
> including user space %cs (USER_CS/USER32_CS) and %ds/%es/%ss (USER_DS).
> Perhaps we could bump down segments 23-28 by one and put it as 23, that
> way the CPU_NUMBER segment would always be at %ss+80 for the default
> (flat, initial) user space %ss.  (We want to specify using %ss rather
> than %ds, because it is less likely to be changed and because 64 bits,
> too, have %ss defined, but not %ds.)
> 
> <action item>
> 
> Rename the x86-64 segment to CPU_NUMBER, fixing the naming conflict.
> Add 1 to GDT entry numbers 23-28 for i386 (all of these are
> kernel-internal segments and so have no impact on user space).
> Add i386 CPU_NUMBER equivalent to x86-64 at GDT entry 23.
> Document the above relationship between segments.
> 
> OK, everyone?
> 
> 	-hpa
> 

Yes GDT_ENTRY_PER_CPU and GDT_ENTRY_PERCPU meaning two totally different things is really confusing,
the proposal seems ok to me!

--Mika