linux-kernel - Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

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Message-ID: <d59ac0eb-e65a-a46f-886e-6df80a2b142f@redhat.com>
Date:   Tue, 17 Dec 2019 13:19:05 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Christophe de Dinechin <dinechin@...hat.com>
Cc:     Peter Xu <peterx@...hat.com>,
        Christophe de Dinechin <christophe.de.dinechin@...il.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

On 17/12/19 13:16, Christophe de Dinechin wrote:
> 
> 
>> On 14 Dec 2019, at 08:57, Paolo Bonzini <pbonzini@...hat.com> wrote:
>>
>> On 13/12/19 21:23, Peter Xu wrote:
>>>> What is the benefit of using u16 for that? That means with 4K pages, you
>>>> can share at most 256M of dirty memory each time? That seems low to me,
>>>> especially since it's sufficient to touch one byte in a page to dirty it.
>>>>
>>>> Actually, this is not consistent with the definition in the code ;-)
>>>> So I'll assume it's actually u32.
>>> Yes it's u32 now.  Actually I believe at least Paolo would prefer u16
>>> more. :)
>>
>> It has to be u16, because it overlaps the padding of the first entry.
> 
> Wow, now that’s subtle.
> 
> That definitely needs a union with the padding to make this explicit.
> 
> (My guess is you do that to page-align the whole thing and avoid adding a
> page just for the counters)

Yes, that was the idea but Peter decided to scrap it. :)

Paolo

>>
>> Paolo
>>
>>> I think even u16 would be mostly enough (if you see, the maximum
>>> allowed value currently is 64K entries only, not a big one).  Again,
>>> the thing is that the userspace should be collecting the dirty bits,
>>> so the ring shouldn't reach full easily.  Even if it does, we should
>>> probably let it stop for a while as explained above.  It'll be
>>> inefficient only if we set it to a too-small value, imho.
>>>
>>
>