Re: [PATCH 2/2] KVM: x86: Add missing trace points for RDMSR/WRMSR in emulator path

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 28, 2022, Hou Wenlong wrote:
> Since the RDMSR/WRMSR emulation uses a sepearte emualtor interface,
> the trace points for RDMSR/WRMSR can be added in emulator path like
> normal path.
> 
> Signed-off-by: Hou Wenlong <houwenlong.hwl@antgroup.com>
> ---
>  arch/x86/kvm/x86.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 8df89b9c212f..6e45b20ce9a4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -7908,12 +7908,16 @@ static int emulator_get_msr_with_filter(struct x86_emulate_ctxt *ctxt,
>  	int r;
>  
>  	r = kvm_get_msr_with_filter(vcpu, msr_index, pdata);
> -	if (r) {
> +	if (!r) {
> +		trace_kvm_msr_read(msr_index, *pdata);
> +	} else {
>  		if (kvm_msr_user_space(vcpu, msr_index, KVM_EXIT_X86_RDMSR, 0,
> -				       complete_emulated_rdmsr, r))
> +				       complete_emulated_rdmsr, r)) {
>  			r = X86EMUL_IO_NEEDED;
> -		else
> +		} else {
> +			trace_kvm_msr_read_ex(msr_index);

Drat, I suspected this patch would make adding a helper a mess.  We could use
trace_kvm_msr() directly, using @exit_reason to select between "rdmsr" and "wrmsr",
but I think the end result is less readable and not worth the small amount
of deduplication.  E.g. this is rather hard to read.

	if (r < 0)
		return X86EMUL_UNHANDLEABLE;

	if (r) {
		if (kvm_msr_user_space(vcpu, msr, exit_reason, data, comp, r))
			return X86EMUL_IO_NEEDED;

		trace_kvm_msr(exit_reason == KVM_EXIT_X86_WRMSR,
			      msr, data, true);
		return X86EMUL_PROPAGATE_FAULT;
	}

	trace_kvm_msr(exit_reason == KVM_EXIT_X86_WRMSR, msr, data, false);
	return X86EMUL_CONTINUE;

Aha!  If there "error" paths return directly, then the "else" paths go away and
this is all (IMO) a bit cleaner.  And the diff for this patch should be much
smaller since there won't be any curly brace changes.

How about this for a final product?

static int emulator_get_msr_with_filter(struct x86_emulate_ctxt *ctxt,
					u32 msr_index, u64 *pdata)
{
	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
	int r;

	r = kvm_get_msr_with_filter(vcpu, msr_index, pdata);
	if (r < 0)
		return X86EMUL_UNHANDLEABLE;

	if (r) {
		if (kvm_msr_user_space(vcpu, msr_index, KVM_EXIT_X86_RDMSR, 0,
				       complete_emulated_rdmsr, r))
			return X86EMUL_IO_NEEDED;

		trace_kvm_msr_read_ex(msr_index);
		return X86EMUL_PROPAGATE_FAULT;
	}

	trace_kvm_msr_read(msr_index, *pdata);
	return X86EMUL_CONTINUE;
}

static int emulator_set_msr_with_filter(struct x86_emulate_ctxt *ctxt,
					u32 msr_index, u64 data)
{
	struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt);
	int r;

	r = kvm_set_msr_with_filter(vcpu, msr_index, data);
	if (r < 0)
		return X86EMUL_UNHANDLEABLE;

	if (r) {
		if (kvm_msr_user_space(vcpu, msr_index, KVM_EXIT_X86_WRMSR, data,
				       complete_emulated_msr_access, r))
			return X86EMUL_IO_NEEDED;

		trace_kvm_msr_write_ex(msr_index, data);
		return X86EMUL_PROPAGATE_FAULT;
	}

	trace_kvm_msr_write(msr_index, data);
	return X86EMUL_CONTINUE;
}