Re: [PATCH 5/5] kvm/svm: copy instruction bytes from VMCB

Avi Kivity
2010-12-07T13:33:27Z

On 12/07/2010 12:59 PM, Andre Przywara wrote:
> In case of a nested page fault or an intercepted #PF newer SVM
> implementations provide a copy of the faulting instruction bytes
> in the VMCB.
> Use these bytes to feed the instruction emulator and avoid the costly
> guest instruction fetch in this case.
>
>
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index cfbcbfa..3e3a67e 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -586,6 +586,9 @@ struct kvm_x86_ops {
> void (*write_tsc_offset)(struct kvm_vcpu *vcpu, u64 offset);
>
> void (*get_exit_info)(struct kvm_vcpu *vcpu, u64 *info1, u64 *info2);
> +
> + int (*prefetch_instruction)(struct kvm_vcpu *vcpu);
> +
> const struct trace_print_flags *exit_reasons_str;
> };

How about adding a byte array/len parameter to x86_decode_insn()? It
could be used to prefill the buffer instead of invoking a callback.

>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 6366735..abff8ff 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -525,6 +525,7 @@ static int do_insn_fetch(struct x86_emulate_ctxt *ctxt,
> /* x86 instructions are limited to 15 bytes. */
> if (eip + size - ctxt->eip> 15)
> return X86EMUL_UNHANDLEABLE;
> + kvm_x86_ops->prefetch_instruction(ctxt->vcpu);

Even with the callback, this belongs in x86_decode_insn(), not on every
fetch.

> while (size--) {
> rc = do_fetch_insn_byte(ctxt, ops, eip++, dest++);
> if (rc != X86EMUL_CONTINUE)
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> index 3cf2cef..ed94e9a 100644
> --- a/arch/x86/kvm/svm.c
> +++ b/arch/x86/kvm/svm.c
> @@ -464,6 +464,24 @@ static void skip_emulated_instruction(struct kvm_vcpu *vcpu)
> svm_set_interrupt_shadow(vcpu, 0);
> }
>
> +static int prefetch_instruction(struct kvm_vcpu *vcpu)

svm_prefetch_instruction()

> +{
> + struct vcpu_svm *svm = to_svm(vcpu);
> + uint8_t len;
> + struct fetch_cache *fetch;
> +
> + len = svm->vmcb->control.insn_len& 0x0F;
> + if (len == 0)
> + return 1;
> +
> + fetch =&svm->vcpu.arch.emulate_ctxt.decode.fetch;
> + fetch->start = kvm_rip_read(&svm->vcpu);
> + fetch->end = fetch->start + len;
> + memcpy(fetch->data, svm->vmcb->control.insn_bytes, len);
> +
> + return 0;
> +}

svm code shouldn't reach in so deep into the emulator privates.

--
error compiling committee.c: too many arguments to function