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

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Date:   Wed, 20 Jul 2022 18:37:46 +0000
From:   Kechen Lu <kechenl@...dia.com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "chao.gao@...el.com" <chao.gao@...el.com>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        Somdutta Roy <somduttar@...dia.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [RFC PATCH v4 3/7] KVM: x86: Reject disabling of MWAIT
 interception when not allowed



> -----Original Message-----
> From: Sean Christopherson <seanjc@...gle.com>
> Sent: Wednesday, July 20, 2022 10:54 AM
> To: Kechen Lu <kechenl@...dia.com>
> Cc: kvm@...r.kernel.org; pbonzini@...hat.com; chao.gao@...el.com;
> vkuznets@...hat.com; Somdutta Roy <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
> 
> External email: Use caution opening links or attachments
> 
> 
> 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.
>
 
Ack!

> > 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.
> 

Sure I see.

> > ---
> >  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.
> 

Yes, I agree, handling here makes sense.

> [*] https://lore.kernel.org/all/Ytg428sleo7uMRQt@google.com
> 
> > +
> > +     if(!kvm_can_mwait_in_guest())
> 
> Space after the "if".

Ack!