Re: [PATCH 12/12] Accelerate nested SVM by emulating parts of GIF=0 v6

[Avi Kivity <avi@redhat.com>]

Alexander Graf wrote:
> Until this patch we bounced between the VM and the VMM for a couple of
> instructions after CLGI, only to find out that yet another SVM instruction
> follows.
> Since roundtrips are really expensive, it's a lot faster to emulate these
> few instructions. Now we can execute CLGI/VMLOAD/VMRUN on one intercept and
> VMEXIT/VMSAVE on another. This is a major speed improvement!
>
>
>   

Neat trick.  Have you looked at svm.c to see if we can cut out 
extraneous instructions (or move them out of the gif=0 area)?  Could 
mean big savings.

> +static int nested_svm_emulate(struct vcpu_svm *svm, struct kvm_run *kvm_run)
> +{
> +	int er;
> +	u32 opcode = 0;
> +	unsigned long rip;
> +	unsigned long rip_linear;
> +
> +	svm->vmcb->save.rax = svm->vcpu.arch.regs[VCPU_REGS_RAX];
> +	svm->vmcb->save.rsp = svm->vcpu.arch.regs[VCPU_REGS_RSP];
> +	svm->vmcb->save.rip = svm->vcpu.arch.regs[VCPU_REGS_RIP];
> +	rip = svm->vcpu.arch.regs[VCPU_REGS_RIP];
> +	rip_linear = rip + svm_seg(&svm->vcpu, VCPU_SREG_CS)->base;
> +
> +	er = emulator_read_std(rip_linear, (void *)&opcode, 3, &svm->vcpu);
> +	if (er != X86EMUL_CONTINUE)
> +		return er;
> +	er = EMULATE_FAIL;
> +
> +	switch (opcode) {
> +		case 0xda010f:
> +			vmload_interception(svm, kvm_run);
> +			er = EMULATE_DONE;
> +			break;
> +		case 0xd8010f:
> +			vmrun_interception(svm, kvm_run);
> +			er = EMULATE_DONE;
> +			break;
> +		case 0xdb010f:
> +			vmsave_interception(svm, kvm_run);
> +			er = EMULATE_DONE;
> +			break;
> +		case 0xdc010f:
> +			stgi_interception(svm, kvm_run);
> +			er = EMULATE_DONE;
> +			break;
> +		default:
> +			nsvm_printk("NSVM: Opcode %x unknown\n", opcode);
> +	}
> +
> +	nsvm_printk("NSVM: svm emul at 0x%lx -> %d\n", rip, er);
> +
> +	return er;
> +}
> +
>   

Move to the regular x86 emulator, so if we extend it with debug flag 
support, privilege checking, etc, we get that for svm as well.

>  static int nested_svm_vmexit_real(struct vcpu_svm *svm, void *arg1,
>  				  void *arg2, void *opaque)
>  {
> @@ -1551,6 +1600,9 @@ static int nested_svm_vmexit(struct vcpu_svm *svm)
>  	kvm_mmu_reset_context(&svm->vcpu);
>  	kvm_mmu_load(&svm->vcpu);
>  
> +	/* KVM calls vmsave after vmrun, so let's run it now if we can */
> +	nested_svm_emulate(svm, NULL);
> +
>   

Will also call stgi eventually, so it may make sense to loop here too.


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