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

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Message-ID: <b5d7cb70-a373-22aa-3e1b-1018799f9293@amd.com>
Date: Mon, 2 Dec 2024 14:33:21 -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 12/2/24 14:13, Tom Lendacky wrote:
> 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;

Until I saw the next patch...  nevermind.

Thanks,
Tom

> 
>> ---
>>  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);