linux-kernel - Re: [PATCH v8 07/15] KVM: pfncache: include page offset in uhva and use it consistently

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Message-ID: <b2728518-f490-4dab-a7c5-fba607352a48@xen.org>
Date:   Wed, 22 Nov 2023 15:52:58 +0000
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Xu Yilun <yilun.xu@...ux.intel.com>
Cc:     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>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v8 07/15] KVM: pfncache: include page offset in uhva and
 use it consistently

On 22/11/2023 15:42, David Woodhouse wrote:
> On Wed, 2023-11-22 at 22:27 +0800, Xu Yilun wrote:
>> On Wed, Nov 22, 2023 at 09:12:18AM +0000, David Woodhouse wrote:
>>> On Wed, 2023-11-22 at 16:54 +0800, Xu Yilun wrote:
>>>>
>>>>> @@ -259,13 +258,25 @@ static int __kvm_gpc_refresh(struct gfn_to_pfn_cache *gpc, gpa_t gpa,
>>>>>                          ret = -EFAULT;
>>>>>                          goto out;
>>>>>                  }
>>>>> +
>>>>> +               hva_change = true;
>>>>> +       } else {
>>>>> +               /*
>>>>> +                * No need to do any re-mapping if the only thing that has
>>>>> +                * changed is the page offset. Just page align it to allow the
>>>>> +                * new offset to be added in.
>>>>
>>>> I don't understand how the uhva('s offset) could be changed when both gpa and
>>>> slot are not changed. Maybe I have no knowledge of xen, but in later
>>>> patch you said your uhva would never change...
>>>
>>> It doesn't change on a normal refresh with kvm_gpc_refresh(), which is
>>> just for revalidation after memslot changes or MMU invalidation.
>>>
>>> But it can change if the gpc is being reinitialized with a new address
>>> (perhaps because the guest has made another hypercall to set the
>>> address, etc.)
>>>
>>> That new address could happen to be in the *same* page as the previous
>>
>> In this case, the lower bits of new gpa should be different to gpc->gpa,
>> so will hit "if (gpc->gpa != gpa ...)" branch.
> 
> I think that 'if (gpc->gpa != gpa); branch is also gratuitously
> refreshing when it doesn't need to; it only needs to refresh if the
> *aligned* gpas don't match.
> 

I did look at that but decided that gfn_to_hva_memslot() was 
sufficiently lightweight that it was not really worth optimising.

> But it was like that already, so I won't heckle Paul any further :)

I appreciate it! :-)

   Paul