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

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Date:   Sat, 14 Dec 2019 11:26:44 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     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 Sat, Dec 14, 2019 at 08:57:26AM +0100, Paolo Bonzini 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.

Hmm, could you explain?

Note that here what Christophe commented is on dirty_index,
reset_index of "struct kvm_dirty_ring", so imho it could really be
anything we want as long as it can store a u32 (which is the size of
the elements in kvm_dirty_ring_indexes).

If you were instead talking about the previous union definition of
"struct kvm_dirty_gfns" rather than "struct kvm_dirty_ring", iiuc I've
moved those indices out of it and defined kvm_dirty_ring_indexes which
we expose via kvm_run, so we don't have that limitation as well any
more?

-- 
Peter Xu