Message-ID: <4C434223.8030202@redhat.com>
Date: Sun, 18 Jul 2010 21:04:19 +0300
From: Avi Kivity <avi@redhat.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
CC: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
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Subject: Re: [patch 1/2] x86_64 page fault NMI-safe
References: <20100714154923.947138065@efficios.com>	<20100714155804.049012415@efficios.com>	<AANLkTiml2uwYqQayTKjMN2gI3LnjVFpwxXkv8GN3McEE@mail.gmail.com>	<20100714170617.GB4955@Krystal>	<AANLkTinLB3gQNKFk9QRfBS8YEfxL3qxZDFw7vWHDOnmL@mail.gmail.com>	<20100714203940.GC22096@Krystal>	<AANLkTinM0eFnreNB5l_6f20oSGBchqAtYPb57tM01kUB@mail.gmail.com>	<20100714222115.GA30122@Krystal>	<AANLkTikLG4xVYfvuMtbuzsqcnzo1ut4dFAm6piqy2Lh7@mail.gmail.com>	<AANLkTin21xBD-BFY4ZX2BPRFnT5-LkgGugvWsvTEP4Mr@mail.gmail.com>	<4C42DF9A.5090908@redhat.com> <AANLkTin1noI6IUfuQsbAgVVzj2rSOhm-Cx3WpsMBgdJ6@mail.gmail.com>
In-Reply-To: <AANLkTin1noI6IUfuQsbAgVVzj2rSOhm-Cx3WpsMBgdJ6@mail.gmail.com>
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On 07/18/2010 08:36 PM, Linus Torvalds wrote:
> On Sun, Jul 18, 2010 at 4:03 AM, Avi Kivity<avi@redhat.com>  wrote:
>    
>> By trading off some memory, we don't need this trickery.  We can allocate
>> two nmi stacks, so the code becomes:
>>      
> I really don't think you need even that. See earlier in the discussion
> about how we could just test %rsp itself. Which makes all the %rip
> testing totally unnecessary, because we don't even need any flags,and
> we have no races because %rsp is atomically changed with taking the
> exception.
>
> Lookie here, the %rsp comparison really isn't that hard:
>
>    nmi:
>        pushq %rax
>        pushq %rdx
>        movq %rsp,%rdx          # current stack top
>        movq 40(%rsp),%rax   # old stack top
>        xor %rax,%rdx              # same 8kB aligned area?
>        shrq $13,%rdx             # ignore low 13 bits
>        je it_is_a_nested_nmi   # looks nested..
>
>    

...

> doesn't that look pretty simple?
>
>    

Too simple - an MCE will switch to its own stack, failing the test.  Now 
that we have correctable MCEs, that's not a good idea.

So the plain everyday sequence

   NMI
   #PF
   MCE (uncompleted)
   NMI

will fail.

Plus, even in the non-nested case, you have to copy the stack frame, or 
the nested NMI will corrupt it.  With stack switching, the nested NMI is 
allocated its own frame.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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