On 2011-08-29 12:59, Avi Kivity wrote:
> On 08/29/2011 01:24 PM, Jan Kiszka wrote:
>>>
>>> static int handle_apic_access(struct kvm_vcpu *vcpu)
>>> {
>>> + unsigned long exit_qualification = vmcs_readl(EXIT_QUALIFICATION);
>>> + int access_type, offset;
>>> +
>>> + access_type = (exit_qualification>> 12)& 0xf;
>>> + offset = exit_qualification& 0xfff;
>>> + /*
>>> + * Sane guest uses MOV instead of string operations to
>>> + * write EOI, with written value not cared. So make a
>>> + * short-circuit here by avoiding heavy instruction
>>> + * emulation.
>>> + */
>>
>> Is there no cheap way to validate this assumption and fall back to the
>> slow path in case it doesn't apply? E.g. reading the first instruction
>> byte and matching it against a whitelist? Even if the ignored scenarios
>> are highly unlikely, I think we so far tried hard to provide both fast
>> and accurate results to the guest in all cases.
>>
>
> 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.
Jan
--
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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