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[70.31.27.79]) by smtp.gmail.com with ESMTPSA id u5-20020a37ab05000000b006b8e8c657ccsm6521149qke.117.2022.09.23.15.34.13 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 23 Sep 2022 15:34:14 -0700 (PDT) Date: Fri, 23 Sep 2022 18:34:12 -0400 From: Peter Xu To: Paolo Bonzini Subject: Re: [PATCH 2/6] KVM: Add KVM_CAP_DIRTY_LOG_RING_ORDERED capability and config option Message-ID: References: <20220922170133.2617189-1-maz@kernel.org> <20220922170133.2617189-3-maz@kernel.org> <87czbmjhbh.wl-maz@kernel.org> MIME-Version: 1.0 In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Disposition: inline Cc: Catalin Marinas , kvm , Marc Zyngier , Andrew Jones , Will Deacon , Shan Gavin , Ben Gardon , David Matlack , Zhenyu Zhang , Shuah Khan , KVM ARM X-BeenThere: kvmarm@lists.cs.columbia.edu X-Mailman-Version: 2.1.14 Precedence: list List-Id: Where KVM/ARM decisions are made List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: kvmarm-bounces@lists.cs.columbia.edu Sender: kvmarm-bounces@lists.cs.columbia.edu On Fri, Sep 23, 2022 at 11:23:24PM +0200, Paolo Bonzini wrote: > Il ven 23 set 2022, 20:26 Peter Xu ha scritto: > > > > > Someone will show up with an old userspace which probes for the sole > > > existing capability, and things start failing subtly. It is quite > > > likely that the userspace code is built for all architectures, > > > > I didn't quite follow here. Since both kvm/qemu dirty ring was only > > supported on x86, I don't see the risk. > > Say you run a new ARM kernel on old userspace, and the new kernel uses > KVM_CAP_DIRTY_LOG_RING. Userspace will try to use the dirty page ring > buffer even though it lacks the memory barriers that were just > introduced in QEMU. > > The new capability means "the dirty page ring buffer is supported and, > by the way, you're supposed to do everything right with respect to > ordering of loads and stores; you can't get away without it like you > could on x86". I understand now, thanks both. -- Peter Xu _______________________________________________ kvmarm mailing list kvmarm@lists.cs.columbia.edu https://lists.cs.columbia.edu/mailman/listinfo/kvmarm