Re: KVM: VMX: optimize APIC EOI

[Avi Kivity <avi@redhat.com>]

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.