Re: [PATCH 8/8] x86, cleanups: Simplify sync_core() in the case ofno CPUID

[H. Peter Anvin]

On 11/30/2012 09:01 AM, Linus Torvalds wrote:
> On Thu, Nov 29, 2012 at 1:24 PM, H. Peter Anvin <hpa@xxxxxxxxxxxxxxx> wrote:
> > Thinking about it some more, there is another reason to not do this,
> > which is that we don't want this particular CPUID to be paravirtualized;
> > we're after the synchronizing side effect, not the CPUID return value
> > itself.
> >
> > So let's leave it as a primitive; it gets too confusing otherwise.
>
> Hmm. The virtualization issue brings up another point: do we *really*
> want to use cpuid for serialization at all?

Well, the grand total of serializing instructions are:

INVD, INVEPT, INVLPG, INVVPID, LGDT, LIDT, LLDT, LTR, MOV to CR, MOV to 
DR, WBINVD, WRMSR, CPUID, IRET, RSM.

It doesn't really leave a lot of wiggle room, and in the microcode case, 
the use of CPUID level 1 is actually mandated (presumably to get a 
uniform sequence for validation purposes.)

> From all of the above, the alternatives case is kinda relevant for virt
> where we do text_poke_early in a loop for every alternative section
> so this could pile up to a bunch of vmexits depending on the emulated
> hardware. Might be worth a replacement if it is noticeable in guests.

This is still boot time, and I really doubt it is measurable in the long 
run.  Yes, exists suck, but at least CPUID is generally a quick exit, 
since all the relevant state is in registers.

	-hpa

--
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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