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

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Message-Id: <E167A793-B42A-422D-8D46-B992CB6EBE69@redhat.com>
Date:   Tue, 17 Dec 2019 13:16:31 +0100
From:   Christophe de Dinechin <dinechin@...hat.com>
To:     Paolo Bonzini <pbonzini@...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 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)

> 
> 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.
>> 
>