linux-kernel - Re: [PATCH v4 4/6] KVM: x86: Bump hypercall stat prior to fully completing hypercall

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Message-ID: <30aa4461-b5bb-a5e2-4a1d-c02d88a2c916@amd.com>
Date: Mon, 2 Dec 2024 14:13:05 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Binbin Wu <binbin.wu@...ux.intel.com>,
 Isaku Yamahata <isaku.yamahata@...el.com>, Kai Huang <kai.huang@...el.com>,
 Xiaoyao Li <xiaoyao.li@...el.com>
Subject: Re: [PATCH v4 4/6] KVM: x86: Bump hypercall stat prior to fully
 completing hypercall

On 11/27/24 18:43, Sean Christopherson wrote:
> Increment the "hypercalls" stat for KVM hypercalls as soon as KVM knows
> it will skip the guest instruction, i.e. once KVM is committed to emulating
> the hypercall.  Waiting until completion adds no known value, and creates a
> discrepancy where the stat will be bumped if KVM exits to userspace as a
> result of trying to skip the instruction, but not if the hypercall itself
> exits.
> 
> Handling the stat in common code will also avoid the need for another
> helper to dedup code when TDX comes along (TDX needs a separate completion
> path due to GPR usage differences).
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

There's a comment in the KVM_HC_MAP_GPA_RANGE case that reads:

	/* stat is incremented on completion. */

that should probably be deleted, but otherwise:

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

Also, if you want, you could get rid of the 'out' label, too, by doing:

	if (cpl)
		return -KVM_EPERM;

> ---
>  arch/x86/kvm/x86.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 13fe5d6eb8f3..11434752b467 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9979,7 +9979,6 @@ static int complete_hypercall_exit(struct kvm_vcpu *vcpu)
>  	if (!is_64_bit_hypercall(vcpu))
>  		ret = (u32)ret;
>  	kvm_rax_write(vcpu, ret);
> -	++vcpu->stat.hypercalls;
>  	return kvm_skip_emulated_instruction(vcpu);
>  }
>  
> @@ -9990,6 +9989,8 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>  {
>  	unsigned long ret;
>  
> +	++vcpu->stat.hypercalls;
> +
>  	trace_kvm_hypercall(nr, a0, a1, a2, a3);
>  
>  	if (!op_64_bit) {
> @@ -10070,7 +10071,6 @@ unsigned long __kvm_emulate_hypercall(struct kvm_vcpu *vcpu, unsigned long nr,
>  	}
>  
>  out:
> -	++vcpu->stat.hypercalls;
>  	return ret;
>  }
>  EXPORT_SYMBOL_GPL(__kvm_emulate_hypercall);