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From:   Paul Durrant <xadimgnik@gmail.com>
Message-ID: <69b2a8ae-fcae-75b8-4b2a-ca75bbd273f0@xen.org>
Date:   Thu, 14 Sep 2023 11:34:56 +0200
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Subject: Re: [PATCH 2/8] KVM: pfncache: add a mark-dirty helper
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To:     David Woodhouse <dwmw2@infradead.org>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Cc:     Paul Durrant <pdurrant@amazon.com>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org
References: <20230914084946.200043-1-paul@xen.org>
 <20230914084946.200043-3-paul@xen.org>
 <87b3f6713a7c6aa57adc89b6c47be3e1511f66ca.camel@infradead.org>
Organization: Xen Project
In-Reply-To: <87b3f6713a7c6aa57adc89b6c47be3e1511f66ca.camel@infradead.org>
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On 14/09/2023 10:21, David Woodhouse wrote:
> On Thu, 2023-09-14 at 08:49 +0000, Paul Durrant wrote:
>> --- a/arch/x86/kvm/xen.c
>> +++ b/arch/x86/kvm/xen.c
>> @@ -430,14 +430,13 @@ static void kvm_xen_update_runstate_guest(struct kvm_vcpu *v, bool atomic)
>>                  smp_wmb();
>>          }
>>   
>> -       if (user_len2)
>> +       if (user_len2) {
>> +               kvm_gpc_mark_dirty(gpc2);
>>                  read_unlock(&gpc2->lock);
>> +       }
>>   
>> +       kvm_gpc_mark_dirty(gpc1);
>>          read_unlock_irqrestore(&gpc1->lock, flags);
>> -
>> -       mark_page_dirty_in_slot(v->kvm, gpc1->memslot, gpc1->gpa >> PAGE_SHIFT);
>> -       if (user_len2)
>> -               mark_page_dirty_in_slot(v->kvm, gpc2->memslot, gpc2->gpa >> PAGE_SHIFT);
>>   }
>>   
>>   void kvm_xen_update_runstate(struct kvm_vcpu *v, int state)
> 
> ISTR there was a reason why the mark_page_dirty_in_slot() was called
> *after* unlocking. Although now I say it, that seems wrong... is that
> because the spinlock is only protecting the uHVA→kHVA mapping, while
> the memslot/gpa are going to remain valid even after unlock, because
> those are protected by sRCU?

Without the lock you could see an inconsistent GPA and memslot so I 
think you could theoretically calculate a bogus rel_gfn and walk off the 
end of the dirty bitmap. Hence moving the call inside the lock while I 
was in the neighbourhood seemed like a good idea. I could call it out in 
the commit comment if you'd like.

   Paul