linux-kernel - Re: [PATCH 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU

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Date:   Thu, 1 Apr 2021 19:02:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
        Peter Xu <peterx@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Peter Feiner <pfeiner@...gle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jim Mattson <jmattson@...gle.com>,
        Yulei Zhang <yulei.kernel@...il.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 12/13] KVM: x86/mmu: Fast invalidation for TDP MMU

On 01/04/21 18:50, Ben Gardon wrote:
>> This is just cosmetic, but I'd prefer to keep the call to
>> kvm_tdp_mmu_invalidate_roots at the original place, so that it's clear
>> in the next patch that it's two separate parts because of the different
>> locking requirements.
> I'm not sure exactly what you mean and I could certainly do a better
> job explaining in the commit description, but it's actually quite
> important that kvm_tdp_mmu_invalidate_roots at least precede
> kvm_zap_obsolete_pages as kvm_zap_obsolete_pages drops the lock and
> yields. If kvm_tdp_mmu_invalidate_roots doesn't go first then vCPUs
> could wind up dropping their ref on an old root and then picking it up
> again before the last root had a chance to drop its ref.
> Explaining in the description that kvm_tdp_mmu_zap_all is being
> dropped because it is no longer necessary (as opposed to being moved)
> might help make that cleaner.

No, what would help is the remark you just made about 
kvm_zap_obsolete_pages yielding.  But that doesn't matter after 13/13 
though, does it?  Perhaps it's easier to just combine them.

Paolo