linux-kernel - Re: [PATCH v2 0/9] KVM: x86/MMU: Optimize disabling dirty logging

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Date:   Tue, 12 Jul 2022 01:37:52 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, Peter Xu <peterx@...hat.com>,
        David Matlack <dmatlack@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        David Dunn <daviddunn@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Junaid Shahid <junaids@...gle.com>
Subject: Re: [PATCH v2 0/9] KVM: x86/MMU: Optimize disabling dirty logging

On Fri, Mar 25, 2022, Paolo Bonzini wrote:
> On 3/21/22 23:43, Ben Gardon wrote:
> > Currently disabling dirty logging with the TDP MMU is extremely slow.
> > On a 96 vCPU / 96G VM it takes ~256 seconds to disable dirty logging
> > with the TDP MMU, as opposed to ~4 seconds with the legacy MMU. This
> > series optimizes TLB flushes and introduces in-place large page
> > promotion, to bring the disable dirty log time down to ~3 seconds.
> > 
> > Testing:
> > Ran KVM selftests and kvm-unit-tests on an Intel Haswell. This
> > series introduced no new failures.
> 
> Thanks, looks good.  The one change I'd make is to just place the outcome of
> build_tdp_shadow_zero_bits_mask() in a global (say tdp_shadow_zero_check) at
> kvm_configure_mmu() time.  The tdp_max_root_level works as a conservative
> choice for the second argument of build_tdp_shadow_zero_bits_mask().
> 
> No need to do anything though, I'll handle this later in 5.19 time (and
> first merge my changes that factor out the constant part of
> vcpu->arch.root_mmu initialization, since this is part of the same ideas).

This fell through the cracks.  Ben is on a long vacation, I'll find my copy of
the Necronomicon and do a bit of resurrection, and address the feedback from v2
along the way.