Re: [PATCH] KVM: APIC: avoid instruction emulation for EOI writes

[Avi Kivity <avi@redhat.com>]

On 08/29/2011 04:55 PM, Jan Kiszka wrote:
> On 2011-08-29 13:11, Avi Kivity wrote:
> >  On 08/29/2011 02:03 PM, Jan Kiszka wrote:
> >>>
> >>>   Just reading the first byte requires a guest page table walk.  This is
> >>>   probably the highest cost in emulation (which also requires a walk for
> >>>   the data access).
> >>
> >>  And what about caching the result of the first walk? Usually, a "sane
> >>  guest" won't have many code pages that issue the EIO.
> >>
> >
> >  There's no way to know when to invalidate the cache.
>
> Set the affected code page read-only?

The virt-phys mapping could change too.  And please, don't think of new 
reasons to write protect pages, they break up my lovely 2M maps.

> >
> >  We could go a bit further, and cache the the whole thing.  On the first
> >  exit, do the entire emulation, and remember %rip.  On the second exit,
> >  if %rip matches, skip directly to kvm_lapic_eoi().
> >
> >  But I don't think it's worth it.  This also has failure modes, and
> >  really, no guest will ever write to EOI with stosl.
>
> ...or add/sub/and/or etc.

Argh, yes, flags can be updated.

Actually, this might work - if we get a read access first as part of the 
RMW, we'll emulate the instruction.  No idea what the hardware does in 
this case.

>   Well, we've done other crazy things in the
> past just to keep even the unlikely case correct. I was just wondering
> if that policy changed.

I can't answer yes to that question.  But I see no way to make it work 
both fast and correct.

>
> However, I just realized that user space is able to avoid this
> inaccuracy for potentially insane guests by not using in-kernel
> irqchips. So we have at least a knob.

Could/should have a flag to disable this in the kernel as well.

-- 
error compiling committee.c: too many arguments to function