linux-kernel - Re: [RFC PATCH v4 3/7] KVM: x86: Reject disabling of MWAIT interception when not allowed

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Message-ID: <YthBJsKOhgHfVs1u@google.com>
Date:   Wed, 20 Jul 2022 17:53:42 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kechen Lu <kechenl@...dia.com>
Cc:     kvm@...r.kernel.org, pbonzini@...hat.com, chao.gao@...el.com,
        vkuznets@...hat.com, somduttar@...dia.com,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v4 3/7] KVM: x86: Reject disabling of MWAIT
 interception when not allowed

On Tue, Jun 21, 2022, Kechen Lu wrote:
> From: Sean Christopherson <seanjc@...gle.com>
> 
> Reject KVM_CAP_X86_DISABLE_EXITS if userspace attempts to disable MWAIT
> exits and KVM previously reported (via KVM_CHECK_EXTENSION) that MWAIT is
> not allowed in guest, e.g. because it's not supported or the CPU doesn't
> have an aways-running APIC timer.
> 
> Fixes: 4d5422cea3b6 ("KVM: X86: Provide a capability to disable MWAIT intercepts")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> Co-developed-by: Kechen Lu <kechenl@...dia.com>

Needs your SOB.

> Suggested-by: Chao Gao <chao.gao@...el.com>

For code review feedback of this nature, adding Suggested-by isn't appropriate.
Suggested-by is for when the idea of the patch itself was suggested by someone,
where as Chao's feedback was a purely mechanical change.

> ---
>  arch/x86/kvm/x86.c | 20 +++++++++++++-------
>  1 file changed, 13 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index b419b258ed90..6ec01362a7d8 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -4199,6 +4199,16 @@ static inline bool kvm_can_mwait_in_guest(void)
>  		boot_cpu_has(X86_FEATURE_ARAT);
>  }
>  
> +static u64 kvm_get_allowed_disable_exits(void)
> +{
> +	u64 r = KVM_X86_DISABLE_VALID_EXITS;

In v3 I "voted" to keep the switch to KVM_X86_DISABLE_VALID_EXITS in the next
patch[*], but seeing the result I 100% agree it's better to handle it here since
the "enable" patch previously used KVM_X86_DISABLE_VALID_EXITS.

[*] https://lore.kernel.org/all/Ytg428sleo7uMRQt@google.com

> +
> +	if(!kvm_can_mwait_in_guest())

Space after the "if".