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Tsirkin" CC: "virtio-dev@lists.oasis-open.org" , "cohuck@redhat.com" , "virtio-comment@lists.oasis-open.org" , Shahaf Shuler Thread-Topic: [PATCH 00/11] Introduce transitional mmr pci device Thread-Index: AQHZY1tF5ObSeHDFk0eWHj0gIzL7fK8UdveAgAD15oCABER6gIAAAE0wgAADeYCAAAA9cA== Date: Mon, 3 Apr 2023 15:16:53 +0000 Message-ID: References: <20230330225834.506969-1-parav@nvidia.com> <20230331024500-mutt-send-email-mst@kernel.org> <0dcd9907-4bb0-ef0d-678d-5bc8f0ded9ec@nvidia.com> <20230403105050-mutt-send-email-mst@kernel.org> <20230403110320-mutt-send-email-mst@kernel.org> In-Reply-To: <20230403110320-mutt-send-email-mst@kernel.org> Accept-Language: en-US Content-Language: en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: authentication-results: dkim=none (message not signed) header.d=none;dmarc=none action=none header.from=nvidia.com; x-ms-publictraffictype: Email x-ms-traffictypediagnostic: PH0PR12MB5481:EE_|DM4PR12MB6302:EE_ x-ms-office365-filtering-correlation-id: ea88ac9c-57ae-486a-795f-08db345674e8 x-ms-exchange-senderadcheck: 1 x-ms-exchange-antispam-relay: 0 x-microsoft-antispam: BCL:0; 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charset="us-ascii" Content-Transfer-Encoding: quoted-printable MIME-Version: 1.0 X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-AuthAs: Internal X-MS-Exchange-CrossTenant-AuthSource: PH0PR12MB5481.namprd12.prod.outlook.com X-MS-Exchange-CrossTenant-Network-Message-Id: ea88ac9c-57ae-486a-795f-08db345674e8 X-MS-Exchange-CrossTenant-originalarrivaltime: 03 Apr 2023 15:16:53.6040 (UTC) X-MS-Exchange-CrossTenant-fromentityheader: Hosted X-MS-Exchange-CrossTenant-id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-mailboxtype: HOSTED X-MS-Exchange-CrossTenant-userprincipalname: 8WqUOOOwkUpHXnzGkXqaylFB5haA17ccgE2e8CLCDjMQbwBVdiYVnUlXq2h/bdovaM08nU7LY3fWBzJ4DGQpzQ== X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM4PR12MB6302 Subject: [virtio-comment] RE: [PATCH 00/11] Introduce transitional mmr pci device > From: Michael S. Tsirkin > Sent: Monday, April 3, 2023 11:07 AM > > > OTOH it is presumably required for scalability anyway, no? > > No. > > Most new generation SIOV and SR-IOV devices operate without any para- > virtualization. >=20 > Don't see the connection to PV. You need an emulation layer in the host i= f you > want to run legacy guests. Looks like it could do transport vq just as we= ll. > Transport vq for legacy MMR purpose seems fine with its latency and DMA ove= rheads. Your question was about "scalability". After your latest response, I am unclear what "scalability" means. Do you mean saving the register space in the PCI device? If yes, than, no for legacy guests for scalability it is not required, beca= use the legacy register is subset of 1.x. =20 > > > And presumably it can all be done in firmware ... > > > Is there actual hardware that can't implement transport vq but is > > > going to implement the mmr spec? > > > > > Nvidia and Marvell DPUs implement MMR spec. >=20 > Hmm implement it in what sense exactly? > Do not follow the question. The proposed series will be implemented as PCI SR-IOV devices using MMR spe= c. =20 > > Transport VQ has very high latency and DMA overheads for 2 to 4 bytes > read/write. >=20 > How many of these 2 byte accesses trigger from a typical guest? >=20 Mostly during the VM boot time. 20 to 40 registers read write access. > > And before discussing "why not that approach", lets finish reviewing "t= his > approach" first. >=20 > That's a weird way to put it. We don't want so many ways to do legacy if = we can > help it. Sure, so lets finish the review of current proposal details. At the moment=20 a. I don't see any visible gain of transport VQ other than device reset par= t I explained. b. it can be a way with high latency, DMA overheads on the virtqueue for re= ad/writes for small access. This publicly archived list offers a means to provide input to the=0D OASIS Virtual I/O Device (VIRTIO) TC.=0D =0D In order to verify user consent to the Feedback License terms and=0D to minimize spam in the list archive, subscription is required=0D before posting.=0D =0D Subscribe: virtio-comment-subscribe@lists.oasis-open.org=0D Unsubscribe: virtio-comment-unsubscribe@lists.oasis-open.org=0D List help: virtio-comment-help@lists.oasis-open.org=0D List archive: https://lists.oasis-open.org/archives/virtio-comment/=0D Feedback License: https://www.oasis-open.org/who/ipr/feedback_license.pdf=0D List Guidelines: https://www.oasis-open.org/policies-guidelines/mailing-lists=0D Committee: https://www.oasis-open.org/committees/virtio/=0D Join OASIS: https://www.oasis-open.org/join/