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[79.178.123.8]) by smtp.gmail.com with ESMTPSA id z6sm6435422wml.41.2020.08.06.03.00.22 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 06 Aug 2020 03:00:23 -0700 (PDT) Date: Thu, 6 Aug 2020 06:00:21 -0400 From: "Michael S. Tsirkin" To: Jason Wang Cc: linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org Subject: Re: [PATCH v2 19/24] vdpa: make sure set_features in invoked for legacy Message-ID: <20200806055818-mutt-send-email-mst@kernel.org> References: <20200803205814.540410-1-mst@redhat.com> <20200803205814.540410-20-mst@redhat.com> <9e47d227-f220-4651-dcb9-7a11f059a715@redhat.com> <20200805073929-mutt-send-email-mst@kernel.org> <20200806015112-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Aug 06, 2020 at 03:27:38PM +0800, Jason Wang wrote: > > On 2020/8/6 下午1:53, Michael S. Tsirkin wrote: > > On Thu, Aug 06, 2020 at 11:23:05AM +0800, Jason Wang wrote: > > > On 2020/8/5 下午7:40, Michael S. Tsirkin wrote: > > > > On Wed, Aug 05, 2020 at 02:14:07PM +0800, Jason Wang wrote: > > > > > On 2020/8/4 上午5:00, Michael S. Tsirkin wrote: > > > > > > Some legacy guests just assume features are 0 after reset. > > > > > > We detect that config space is accessed before features are > > > > > > set and set features to 0 automatically. > > > > > > Note: some legacy guests might not even access config space, if this is > > > > > > reported in the field we might need to catch a kick to handle these. > > > > > I wonder whether it's easier to just support modern device? > > > > > > > > > > Thanks > > > > Well hardware vendors are I think interested in supporting legacy > > > > guests. Limiting vdpa to modern only would make it uncompetitive. > > > > > > My understanding is that, IOMMU_PLATFORM is mandatory for hardware vDPA to > > > work. > > Hmm I don't really see why. Assume host maps guest memory properly, > > VM does not have an IOMMU, legacy guest can just work. > > > Yes, guest may not set IOMMU_PLATFORM. > > > > > > Care explaining what's wrong with this picture? > > > The problem is virtio_vdpa, without IOMMU_PLATFORM it uses PA which can not > work if IOMMU is enabled. > > Thanks So that's a virtio_vdpa limitation. In the same way, if a device does not have an on-device iommu *and* is not behind an iommu, then vdpa can't bind to it. But this virtio_vdpa specific hack does not belong in a generic vdpa code. > > > > > > > > So it can only work for modern device ... > > > > > > Thanks > > > > > > > > > > > > > >