linux-kernel - Re: [PATCH v3 01/12] Revert "KVM: x86/mmu: Allow zap gfn range to operate under the mmu read lock"

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Message-ID: <15a05059-beae-904b-645e-17faf760c261@redhat.com>
Date:   Tue, 3 Aug 2021 19:29:50 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org,
        Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Jim Mattson <jmattson@...gle.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>,
        Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3 01/12] Revert "KVM: x86/mmu: Allow zap gfn range to
 operate under the mmu read lock"

On 03/08/21 17:11, Sean Christopherson wrote:
>> On top of this, I think the CD case (kvm_post_set_cr0) can be changed to use
>> kvm_mmu_zap_all_fast.
> No, because fast zap requires kvm->slots_lock be held.

Yeah, that much I knew. :)  The issue is the usual ordering between 
slots_lock and SRCU.

Paolo

   That could be relaxed by
> reverting ca333add6933 ("KVM: x86/mmu: Explicitly track only a single invalid mmu
> generation") and converting mmu_valid_gen to a u64 (to prevent wrap on 32-bit KVM).
> IMO the extra memory cost, even though it's meager savings when using TDP without
> nested, isn't worth relaxing the rules for fast zap.  Non-coherent DMA isn't very
> common these days, and toggling CR0.CD is a rare guest operation (it'd probably
> never happen if the darn architcture didn't set it on RESET).
>