linux-kernel - Re: [PATCH 3/4] KVM: SVM: Adjust MMIO masks (for caching) before doing SEV(-ES) setup

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Message-ID: <YuQjtLK1uk3/bhK/@google.com>
Date:   Fri, 29 Jul 2022 18:15:16 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Michael Roth <michael.roth@....com>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 3/4] KVM: SVM: Adjust MMIO masks (for caching) before
 doing SEV(-ES) setup

On Fri, Jul 29, 2022, Kai Huang wrote:
> On Thu, 2022-07-28 at 22:17 +0000, Sean Christopherson wrote:
> > Adjust KVM's MMIO masks to account for the C-bit location prior to doing
> > SEV(-ES) setup.  A future patch will consume enable_mmio caching during
> > SEV setup as SEV-ES _requires_ MMIO caching, i.e. KVM needs to disallow
> > SEV-ES if MMIO caching is disabled.
> > 
> > Cc: stable@...r.kernel.org
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/kvm/svm/svm.c | 9 ++++++---
> >  1 file changed, 6 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index aef63aae922d..62e89db83bc1 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -5034,13 +5034,16 @@ static __init int svm_hardware_setup(void)
> >  	/* Setup shadow_me_value and shadow_me_mask */
> >  	kvm_mmu_set_me_spte_mask(sme_me_mask, sme_me_mask);
> >  
> > -	/* Note, SEV setup consumes npt_enabled. */
> > +	svm_adjust_mmio_mask();
> > +
> > +	/*
> > +	 * Note, SEV setup consumes npt_enabled and enable_mmio_caching (which
> > +	 * may be modified by svm_adjust_mmio_mask()).
> > +	 */
> >  	sev_hardware_setup();
> 
> If I am not seeing mistakenly, the code in latest queue branch doesn't consume
> enable_mmio_caching.  It is only added in your later patch.
> 
> So perhaps adjust the comment or merge patches together?

Oooh, I see what you're saying.  I split the patches so that if this movement turns
out to break something then bisection will point directly here, but that's a pretty
weak argument since both patches are tiny.  And taking patch 4 without patch 3,
e.g. in the unlikely event this movement needs to be reverted, is probably worse
than not having patch 4 at all, i.e. having somewhat obvious breakage is better.

So yeah, I'll squash this with patch 4.

Thanks!