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([2001:b07:6468:f312:b509:fc01:ee8a:ca8a]) by smtp.gmail.com with ESMTPSA id c17sm51703396wrr.87.2020.01.21.02.24.56 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 21 Jan 2020 02:24:57 -0800 (PST) Subject: Re: [PATCH v3 12/21] KVM: X86: Implement ring-based dirty memory tracking To: Peter Xu , "Michael S. Tsirkin" Cc: 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 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> From: Paolo Bonzini Message-ID: <4e75a275-6687-2efc-0595-9b993ec300be@redhat.com> Date: Tue, 21 Jan 2020 11:24:56 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.1.1 MIME-Version: 1.0 In-Reply-To: <20200120072915.GD380565@xz-x1> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 20/01/20 08:29, Peter Xu wrote: >>> >>> 00b (invalid GFN) -> >>> 01b (valid gfn published by kernel, which is dirty) -> >>> 1*b (gfn dirty page collected by userspace) -> >>> 00b (gfn reset by kernel, so goes back to invalid gfn) >>> That is 10b and 11b are equivalent. The kernel doesn't read that bit if >>> userspace has collected the page. > Yes "1*b" is good too (IMHO as long as we can define three states for > an entry). However do you want me to change to that? Note that I > still think we need to read the rest of the field (in this case, > "slot" and "gfn") besides the two bits to do re-protect. Should we > trust that unconditionally if writable? I think that userspace would only hurt itself if they do so. As long as the kernel has a trusted copy of the indices, it's okay. We have plenty of bits--x86 limits GFNs to 40 bits (52 bits maximum physical address). However, even on other architectures GFNs are limited to address space size - page shift (64-12). Paolo