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

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Date:   Thu, 14 Sep 2023 15:07:18 +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 13:39, David Woodhouse wrote:
> On Thu, 2023-09-14 at 11:34 +0200, Paul Durrant wrote:
>> 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.
> 
> Yeah, I can't see a reason why it needs to be outside the lock, and as
> you note, there really is a reason why it should be *inside*. Whatever
> reason there was, it either disappeared in the revisions of the gpc
> patch set or it was stupidity on my part in the first place.
> 
> So yeah, let it move inside the lock, call that out in the commit
> message (I did note some of the other commits could have used a 'No
> functional change intended' too, FWIW), and

Ack. Will do.

> 
> Reviewed-by: David Woodhouse <dwmw@...zon.co.uk>
> 

Thanks.

   Paul