linux-kernel - Re: [PATCH v3 12/21] KVM: X86: Implement ring-based dirty memory tracking

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Date:   Tue, 21 Jan 2020 11:25:30 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Peter Xu <peterx@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Christophe de Dinechin <dinechin@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yan Zhao <yan.y.zhao@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Kevin Kevin <kevin.tian@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Lei Cao <lei.cao@...atus.com>
Subject: Re: [PATCH v3 12/21] KVM: X86: Implement ring-based dirty memory
 tracking

On 21/01/20 09:29, Peter Xu wrote:
>>>> If we are short on bits we can just use 1 bit. E.g. set if
>>>> userspace has collected the GFN.
>>> I'm still unsure whether we can use only one bit for this.  Say,
>>> otherwise how does the userspace knows the entry is valid?  For
>>> example, the entry with all zeros ({.slot = 0, gfn = 0}) could be
>>> recognized as a valid dirty page on slot 0 gfn 0, even if it's
>>> actually an unused entry.
>> So I guess the reverse: valid entry has bit set, userspace sets it to
>> 0 when it collects it?
> Right, this seems to work.

Yes, that's okay too.

Paolo