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([2a01:e0a:92b:6460:3f38:a137:6502:95a4]) by smtp.gmail.com with ESMTPSA id l15sm14291193wmi.29.2022.02.15.01.16.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Tue, 15 Feb 2022 01:16:42 -0800 (PST) Subject: Re: [PATCH v3 0/4] virtio-iommu: Support VIRTIO_IOMMU_F_BYPASS_CONFIG To: Cornelia Huck , Jean-Philippe Brucker References: <20220214124356.872985-1-jean-philippe@linaro.org> <87o839s67g.fsf@redhat.com> From: Eric Auger Message-ID: Date: Tue, 15 Feb 2022 10:16:40 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: <87o839s67g.fsf@redhat.com> Authentication-Results: relay.mimecast.com; auth=pass smtp.auth=CUSA124A263 smtp.mailfrom=eric.auger@redhat.com X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Content-Language: en-US Received-SPF: pass client-ip=170.10.129.124; envelope-from=eric.auger@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.083, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, NICE_REPLY_A=-0.001, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-To: eric.auger@redhat.com Cc: lvivier@redhat.com, thuth@redhat.com, mst@redhat.com, qemu-devel@nongnu.org, dgilbert@redhat.com, pasic@linux.ibm.com, pbonzini@redhat.com Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" Hi Connie, On 2/14/22 6:34 PM, Cornelia Huck wrote: > On Mon, Feb 14 2022, Jean-Philippe Brucker wrote: > >> Replace the VIRTIO_IOMMU_F_BYPASS feature with >> VIRTIO_IOMMU_F_BYPASS_CONFIG, which enables a config space bit to switch >> global bypass on and off. >> >> Add a boot-bypass option, which defaults to 'on' to be in line with >> other vIOMMUs and to allow running firmware/bootloader that are unaware >> of the IOMMU. x86 doesn't need a workaround to boot with virtio-iommu >> anymore. >> >> Since v2 [1]: >> * Added the new bypass bits to the migration stream. >> As discussed on the v2 thread, we assume that cross-version >> compatibility is not required for live migration at the moment, so we >> only increase the version number. Patch 2 says: "We add the bypass >> field to the migration stream without introducing subsections, based >> on the assumption that this virtio-iommu device isn't being used in >> production enough to require cross-version migration at the moment >> (all previous version required workarounds since they didn't support >> ACPI and boot-bypass)." >> >> [1] https://lore.kernel.org/qemu-devel/20220127142940.671333-1-jean-philippe@linaro.org/ > One thing that we could do to avoid surprises in the unlikely case that > somebody has a virtio-iommu device and wants to migrate to an older > machine version is to add a migration blocker for the virtio-iommu > device for all compat machines for versions 6.2 or older (i.e. only 7.0 > or newer machine types can have a migratable virtio-iommu device > starting with QEMU 7.0.) Not too complicated to implement, but I'm not > sure whether we'd add too much code to prevent something very unlikely > to happen anyway. I would not insist on it :) As nobody has shout and we are not aware of anybody using the device in production mode yet due to the missing boot bypass feature this series brings, I would be personally in favour of leaving things as is. Now, up to Jean if he wants to go and implement your suggestion. Thanks Eric > > Otherwise, I didn't spot problems in this series. >