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).
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