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

Tom Lendacky
2024-12-02T20:33:32Z

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@google.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@amd.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);