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

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Message-ID: <dba0ecc8-90ae-975f-7a27-3049d6951ba0@redhat.com>
Date:   Fri, 25 Mar 2022 13:00:27 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.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 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).

Paolo