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[109.66.39.109]) by smtp.gmail.com with ESMTPSA id w19sm3881478wmi.0.2020.03.19.10.45.56 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 19 Mar 2020 10:45:58 -0700 (PDT) Date: Thu, 19 Mar 2020 13:45:55 -0400 From: "Michael S. Tsirkin" To: David Hildenbrand Subject: Re: [PATCH v5 07/18] s390x: protvirt: Inhibit balloon when switching to protected mode Message-ID: <20200319133710-mutt-send-email-mst@kernel.org> References: <20200226122038.61481-1-frankja@linux.ibm.com> <20200226122038.61481-8-frankja@linux.ibm.com> <58a51f40-21c7-5737-4f4c-568fdd2477fa@linux.ibm.com> <20200227132402.67a38047.pasic@linux.ibm.com> <8622efeb-1a4a-338f-d363-53818b00d195@redhat.com> MIME-Version: 1.0 In-Reply-To: <8622efeb-1a4a-338f-d363-53818b00d195@redhat.com> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: quoted-printable Content-Disposition: inline X-detected-operating-system: by eggs.gnu.org: GNU/Linux 2.2.x-3.x [generic] [fuzzy] X-Received-From: 216.205.24.74 X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.23 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Janosch Frank , cohuck@redhat.com, qemu-devel@nongnu.org, Halil Pasic , borntraeger@de.ibm.com, qemu-s390x@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On Thu, Mar 19, 2020 at 02:54:11PM +0100, David Hildenbrand wrote: > Why does the balloon driver not support VIRTIO_F_IOMMU_PLATFORM? It is > absolutely not clear to me. The introducing commit mentioned that it > "bypasses DMA". I fail to see that. Well sure one can put the balloon behind an IOMMU. If will shuffle PFN lists through a shared page. Problem is, you can't run an untrusted driver with it since if you do it can corrupt guest memory. And VIRTIO_F_IOMMU_PLATFORM so far meant that you can run a userspace driver. Maybe we need a separate feature bit for this kind of thing where you assume the driver is trusted? Such a bit - unlike VIRTIO_F_IOMMU_PLATFORM - would allow legacy guests ... --=20 MST