linux-kernel - Re: [PATCH v3 1/2] KVM: x86/mmu: Ensure TDP MMU roots are freed after yield

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Message-ID: <2288be2a-b485-408d-b63c-85e00dbc5964@oracle.com>
Date:   Thu, 7 Jan 2021 16:36:32 +0100
From:   "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
To:     Ben Gardon <bgardon@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Peter Shier <pshier@...gle.com>,
        Leo Hou <leohou1402@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH v3 1/2] KVM: x86/mmu: Ensure TDP MMU roots are freed after
 yield

On 07.01.2021 01:19, Ben Gardon wrote:
> Many TDP MMU functions which need to perform some action on all TDP MMU
> roots hold a reference on that root so that they can safely drop the MMU
> lock in order to yield to other threads. However, when releasing the
> reference on the root, there is a bug: the root will not be freed even
> if its reference count (root_count) is reduced to 0.
> 
> To simplify acquiring and releasing references on TDP MMU root pages, and
> to ensure that these roots are properly freed, move the get/put operations
> into another TDP MMU root iterator macro.
> 
> Moving the get/put operations into an iterator macro also helps
> simplify control flow when a root does need to be freed. Note that using
> the list_for_each_entry_safe macro would not have been appropriate in
> this situation because it could keep a pointer to the next root across
> an MMU lock release + reacquire, during which time that root could be
> freed.
> 
> Reported-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
> Suggested-by: Paolo Bonzini <pbonzini@...hat.com>
> Fixes: faaf05b00aec ("kvm: x86/mmu: Support zapping SPTEs in the TDP MMU")
> Fixes: 063afacd8730 ("kvm: x86/mmu: Support invalidate range MMU notifier for TDP MMU")
> Fixes: a6a0b05da9f3 ("kvm: x86/mmu: Support dirty logging for the TDP MMU")
> Fixes: 14881998566d ("kvm: x86/mmu: Support disabling dirty logging for the tdp MMU")
> Signed-off-by: Ben Gardon <bgardon@...gle.com>

Tested this version, too, and it works fine, so:
Tested-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>

Thanks,
Maciej