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

[Jan Kiszka <jan.kiszka@siemens.com>]

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?

> 
> 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. 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.

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.

Jan

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