Re: KVM: VMX: optimize APIC EOI

Avi Kivity
2010-06-15T04:33:16Z

On 06/15/2010 01:30 AM, Marcelo Tosatti wrote:
> Use information provided in exit_qualification to shortcut EOI path.
>
> Reduces EOI latency from 4k to 2k cycles on Nehalem.
>
>

This already came up once - IIRC from Eddie Dong.
>
> 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;
> + if (access_type == 1&& offset == APIC_EOI) {
> + kvm_lapic_set_eoi(vcpu);
> + skip_emulated_instruction(vcpu);
> + return 1;
> + }
> +
> return emulate_instruction(vcpu, 0, 0, 0) == EMULATE_DONE;
> }
>
>

This fails if the instruction is STOS, MOVS, or a RMW instruction as it
doesn't update registers and flags.

We could try to do tricks to detect this, but we already have x2apic for
Linux and the corresponding Hyper-V extension, which improve a lot of
other stuff, at least for newer guests.

\

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.