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
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