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Date: Tue, 22 Apr 2014 10:59:21 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Andrew Lutomirski <amluto@gmail.com>
CC: Linus Torvalds <torvalds@linux-foundation.org>,
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        Alexandre Julliard <julliard@winehq.com>,
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Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*
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On 04/22/2014 10:46 AM, Andrew Lutomirski wrote:
>>
>> That is the whole impact of the IRET path.
>>
>> If using IST for #GP won't cause trouble (ISTs don't nest, so we need to
>> make sure there is absolutely no way we could end up nested) then the
>> rest of the fixup code can go away and we kill the common path
>> exception-handling overhead; the only new overhead is the IST
>> indirection for #GP, which isn't a performance-critical fault (good
>> thing, because untangling #GP faults is a major effort.)
> 
> I'd be a bit nervous about read_msr_safe and friends.  Also, what
> happens if userspace triggers a #GP and the signal stack setup causes
> a page fault?
> 

Yes, #GPs inside the kernel could be a real problem.  MSRs generally
don't trigger #SS.  The second scenario shouldn't be a problem, the #PF
will be delivered on the currently active stack.

On the other hand, doing the espfix fixup only for #GP might be an
alternative, as long as we can convince ourselves that it really is the
only fault that could possibly be delivered on the espfix path.

	-hpa