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Tsirkin" 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 Date: Thu, 03 Apr 2025 09:22:57 +0100 In-Reply-To: <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> <20250403040643-mutt-send-email-mst@kernel.org> Content-Type: multipart/signed; micalg="sha-256"; protocol="application/pkcs7-signature"; boundary="=-Pdiw6S/CK/grIWdI4MBF" User-Agent: Evolution 3.52.3-0ubuntu1 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-SRS-Rewrite: SMTP reverse-path rewritten from by desiato.infradead.org. See http://www.infradead.org/rpr.html --=-Pdiw6S/CK/grIWdI4MBF Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Thu, 2025-04-03 at 04:13 -0400, Michael S. Tsirkin wrote: > 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: > > >=20 > > > Indeed I personally do not exactly get why implement a virtual system > > > without an IOMMU when virtio-iommu is available. > > >=20 > > > I have a feeling it's about lack of windows drivers for virtio-iommu > > > at this point. > >=20 > > 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? >=20 > Is there a blocker here? Only the amount of complexity in what should be a minimal Trusted Compute Base. (And ideally subject to formal methods of proving its correctness too.) And frankly, if we were going to accept a virtio-iommu in the TCB why not just implement enough virtqueue knowledge to build something where the trusted part just snoops on the *actual* e.g. virtio-net device to know which buffers the VMM was *invited* to access, and facilitate that? We looked at doing that. It's awful. > > 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? >=20 > They should. In particular because systems with multiple IOMMUs already > exist. >=20 > > Do the available discovery mechanisms for all the relevant IOMMUs > > even *allow* for that to be expressed? >=20 > 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. Even if it could work in theory, I'll be astonished if it actually works in practice across a wide set of operating systems, and if it *ever* works for Windows. 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