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Tsirkin" To: David Woodhouse Cc: Zhu Lingshan , virtio-comment@lists.linux.dev, hch@infradead.org, Claire Chang , linux-devicetree , Rob Herring , =?iso-8859-1?Q?J=F6rg?= Roedel , iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org, graf@amazon.de Subject: Re: [RFC PATCH 1/3] content: Add VIRTIO_F_SWIOTLB to negotiate use of SWIOTLB bounce buffers Message-ID: <20250403040643-mutt-send-email-mst@kernel.org> References: <20250402112410.2086892-1-dwmw2@infradead.org> <20250402112410.2086892-2-dwmw2@infradead.org> <1966cbf1-a430-480e-a78d-3d6bbcb4ada4@amd.com> <20250403033230-mutt-send-email-mst@kernel.org> <59be937432fe73b5781ecb04aad501ae5a11be23.camel@infradead.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <59be937432fe73b5781ecb04aad501ae5a11be23.camel@infradead.org> On Thu, Apr 03, 2025 at 08:54:45AM +0100, David Woodhouse wrote: > On Thu, 2025-04-03 at 03:34 -0400, Michael S. Tsirkin wrote: > > > > Indeed I personally do not exactly get why implement a virtual system > > without an IOMMU when virtio-iommu is available. > > > > I have a feeling it's about lack of windows drivers for virtio-iommu > > at this point. > > And a pKVM (etc.) implementation of virtio-iommu which would allow the > *trusted* part of the hypervisor to know which guest memory should be > shared with the VMM implementing the virtio device models? Is there a blocker here? > You'd also end up in a situation where you have a virtio-iommu for some > devices, and a real two-stage IOMMU (e.g. SMMU or AMD's vIOMMU) for > other devices. Are guest operating systems going to cope well with > that? They should. In particular because systems with multiple IOMMUs already exist. > Do the available discovery mechanisms for all the relevant IOMMUs > even *allow* for that to be expressed? I think yes. But, it's been a while since I played with this, let me check what works, what does not, and get back to you on this. -- MST