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Tsirkin" Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Christophe de Dinechin , Sean Christopherson , Yan Zhao , Alex Williamson , Jason Wang , Kevin Kevin , Vitaly Kuznetsov , "Dr . David Alan Gilbert" , Lei Cao Subject: Re: [PATCH v3 12/21] KVM: X86: Implement ring-based dirty memory tracking Message-ID: <20200121082925.GB440822@xz-x1> References: <20200109145729.32898-1-peterx@redhat.com> <20200109145729.32898-13-peterx@redhat.com> <20200109110110-mutt-send-email-mst@kernel.org> <20200109191514.GD36997@xz-x1> <22bcd5fc-338c-6b72-2bda-47ba38d7e8ef@redhat.com> <20200119051145-mutt-send-email-mst@kernel.org> <20200120072915.GD380565@xz-x1> <20200120024717-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20200120024717-mutt-send-email-mst@kernel.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 20, 2020 at 02:47:46AM -0500, Michael S. Tsirkin wrote: > On Mon, Jan 20, 2020 at 03:29:15PM +0800, Peter Xu wrote: > > On Sun, Jan 19, 2020 at 05:12:35AM -0500, Michael S. Tsirkin wrote: > > > On Sun, Jan 19, 2020 at 10:09:53AM +0100, Paolo Bonzini wrote: > > > > On 09/01/20 20:15, Peter Xu wrote: > > > > > Regarding dropping the indices: I feel like it can be done, though we > > > > > probably need two extra bits for each GFN entry, for example: > > > > > > > > > > - Bit 0 of the GFN address to show whether this is a valid publish > > > > > of dirty gfn > > > > > > > > > > - Bit 1 of the GFN address to show whether this is collected by the > > > > > user > > > > > > > > We can use bit 62 and 63 of the GFN. > > > > > > 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. Thanks, -- Peter Xu