linux-kernel - Re: [PATCH 2/8] KVM: pfncache: add a mark-dirty helper

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Date:   Thu, 14 Sep 2023 11:34:56 +0200
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Paul Durrant <pdurrant@...zon.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH 2/8] KVM: pfncache: add a mark-dirty helper

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