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

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Message-ID: <bc15650b-df59-f508-1090-21dafc6e8ad1@redhat.com>
Date:   Sat, 14 Dec 2019 08:57:26 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>,
        Christophe de Dinechin <christophe.de.dinechin@...il.com>
Cc:     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 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.

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.
>