linux-kernel - Re: [PATCH] KVM: x86/mmu: Remove KVM MMU write lock when accessing indirect_shadow

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Message-ID: <ZH/PKMmWWgJQdcJQ@google.com>
Date:   Tue, 6 Jun 2023 17:28:24 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Mingwei Zhang <mizhang@...gle.com>
Cc:     Jim Mattson <jmattson@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Ben Gardon <bgardon@...gle.com>
Subject: Re: [PATCH] KVM: x86/mmu: Remove KVM MMU write lock when accessing indirect_shadow_pages

On Tue, Jun 06, 2023, Mingwei Zhang wrote:
> > > Hmm. I agree with both points above, but below, the change seems too
> > > heavyweight. smp_wb() is a mfence(), i.e., serializing all
> > > loads/stores before the instruction. Doing that for every shadow page
> > > creation and destruction seems a lot.
> >
> > No, the smp_*b() variants are just compiler barriers on x86.
> 
> hmm, it is a "lock addl" now for smp_mb(). Check this: 450cbdd0125c
> ("locking/x86: Use LOCK ADD for smp_mb() instead of MFENCE")
> 
> So this means smp_mb() is not a free lunch and we need to be a little
> bit careful.

Oh, those sneaky macros.  x86 #defines __smp_mb(), not the outer helper.  I'll
take a closer look before posting to see if there's a way to avoid the runtime
barrier.