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Subject: Re: [PATCH v2 1/2] KVM: x86/mmu: Ensure TDP MMU roots are freed after
 yield
To:     Ben Gardon <bgardon@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Peter Shier <pshier@google.com>,
        Leo Hou <leohou1402@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>
References: <20210106185951.2966575-1-bgardon@google.com>
 <CANgfPd9g3R7Am=EVf+5o0_WFabqQKjmW0t3mtEHe1rOccLFpTg@mail.gmail.com>
From:   "Maciej S. Szmigiero" <maciej.szmigiero@oracle.com>
Message-ID: <386f6b9e-2590-71b2-196e-1f692460ef80@oracle.com>
Date:   Wed, 6 Jan 2021 23:13:51 +0100
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On 06.01.2021 20:03, Ben Gardon wrote:
> On Wed, Jan 6, 2021 at 10:59 AM Ben Gardon <bgardon@google.com> 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 the TDP MMU root iterator macro. Not all functions which use the macro
>> currently get and put a reference to the root, but adding this behavior is
>> harmless.
>>
>> Moving the get/put operations into the iterator macro also helps
>> simplify control flow when a root does need to be freed. Note that using
>> the list_for_each_entry_unsafe macro would not have been appropriate in
>> this situation because it could keep a reference to the next root across
>> an MMU lock release + reacquire.
>>
>> Reported-by: Maciej S. Szmigiero <maciej.szmigiero@oracle.com>
>> Suggested-by: Paolo Bonzini <pbonzini@redhat.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@google.com>
>> ---
(..)
> I tested v2 with Maciej's test
> (https://urldefense.com/v3/__https://gist.github.com/maciejsszmigiero/890218151c242d99f63ea0825334c6c0__;!!GqivPVa7Brio!NUh8Xbu1YkhSf49HvbyhI-svvPmJXWj9KECqaEd7ZJMKPdz-HdND1sKduH2VpwasEN8Gpg$ ,
> near the bottom of the page) on an Intel Skylake Machine and can
> confirm that v1 failed the test but v2 passes. The problem with v1 was
> that roots were being removed from the list before list_next_entry was
> called, resulting in a bad value.
> 

I've tested the fix now and can confirm, too, that I can no longer
observe any crash.

Thanks,
Maciej