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charset="us-ascii" Content-Transfer-Encoding: 7bit Errors-To: iommu-bounces@lists.linux-foundation.org Sender: "iommu" From: Jason Gunthorpe The KVM mechanism for controlling wbinvd is based on OR of the coherency property of all devices attached to a guest, no matter whether those devices are attached to a single domain or multiple domains. On the other hand, the benefit to using separate domains was that those devices attached to domains supporting enforced cache coherency always mapped with the attributes necessary to provide that feature, therefore if a non-enforced domain was dropped, the associated group removal would re-trigger an evaluation by KVM. In practice however, the only known cases of such mixed domains included an Intel IGD device behind an IOMMU lacking snoop control, where such devices do not support hotplug, therefore this scenario lacks testing and is not considered sufficiently relevant to support. After all, KVM won't take advantage of trying to push a device that could do enforced cache coherency to a dedicated domain vs re-using an existing domain, which is non-coherent. Simplify this code and eliminate the test. This removes the only logic that needed to have a dummy domain attached prior to searching for a matching domain and simplifies the next patches. It's unclear whether we want to further optimize the Intel driver to update the domain coherency after a device is detached from it, at least not before KVM can be verified to handle such dynamics in related emulation paths (wbinvd, vcpu load, write_cr0, ept, etc.). In reality we don't see an usage requiring such optimization as the only device which imposes such non-coherency is Intel GPU which even doesn't support hotplug/hot remove. Signed-off-by: Jason Gunthorpe Reviewed-by: Kevin Tian Reviewed-by: Lu Baolu Signed-off-by: Nicolin Chen --- drivers/vfio/vfio_iommu_type1.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c index c496b7d0b96f..7530f0d727e5 100644 --- a/drivers/vfio/vfio_iommu_type1.c +++ b/drivers/vfio/vfio_iommu_type1.c @@ -2288,9 +2288,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data, * testing if they're on the same bus_type. */ list_for_each_entry(d, &iommu->domain_list, next) { - if (d->domain->ops == domain->domain->ops && - d->enforce_cache_coherency == - domain->enforce_cache_coherency) { + if (d->domain->ops == domain->domain->ops) { iommu_detach_group(domain->domain, group->iommu_group); if (!iommu_attach_group(d->domain, group->iommu_group)) { -- 2.17.1 _______________________________________________ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from NAM12-MW2-obe.outbound.protection.outlook.com (mail-mw2nam12on2086.outbound.protection.outlook.com [40.107.244.86]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id BD04B1870 for ; 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X-OriginatorOrg: Nvidia.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 01 Jul 2022 21:45:07.5802 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: daa3572e-c837-4fed-2d9e-08da5baaf73d X-MS-Exchange-CrossTenant-Id: 43083d15-7273-40c1-b7db-39efd9ccc17a X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=43083d15-7273-40c1-b7db-39efd9ccc17a;Ip=[12.22.5.238];Helo=[mail.nvidia.com] X-MS-Exchange-CrossTenant-AuthSource: CO1NAM11FT038.eop-nam11.prod.protection.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: PH7PR12MB6000 Message-ID: <20220701214452.p2AjVMSZ-vzeR5vwlPmqpzB3Q4ylYBGN31cc9Rdh3gs@z> From: Jason Gunthorpe The KVM mechanism for controlling wbinvd is based on OR of the coherency property of all devices attached to a guest, no matter whether those devices are attached to a single domain or multiple domains. On the other hand, the benefit to using separate domains was that those devices attached to domains supporting enforced cache coherency always mapped with the attributes necessary to provide that feature, therefore if a non-enforced domain was dropped, the associated group removal would re-trigger an evaluation by KVM. In practice however, the only known cases of such mixed domains included an Intel IGD device behind an IOMMU lacking snoop control, where such devices do not support hotplug, therefore this scenario lacks testing and is not considered sufficiently relevant to support. After all, KVM won't take advantage of trying to push a device that could do enforced cache coherency to a dedicated domain vs re-using an existing domain, which is non-coherent. Simplify this code and eliminate the test. This removes the only logic that needed to have a dummy domain attached prior to searching for a matching domain and simplifies the next patches. It's unclear whether we want to further optimize the Intel driver to update the domain coherency after a device is detached from it, at least not before KVM can be verified to handle such dynamics in related emulation paths (wbinvd, vcpu load, write_cr0, ept, etc.). In reality we don't see an usage requiring such optimization as the only device which imposes such non-coherency is Intel GPU which even doesn't support hotplug/hot remove. Signed-off-by: Jason Gunthorpe Reviewed-by: Kevin Tian Reviewed-by: Lu Baolu Signed-off-by: Nicolin Chen --- drivers/vfio/vfio_iommu_type1.c | 4 +--- 1 file changed, 1 insertion(+), 3 deletions(-) diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c index c496b7d0b96f..7530f0d727e5 100644 --- a/drivers/vfio/vfio_iommu_type1.c +++ b/drivers/vfio/vfio_iommu_type1.c @@ -2288,9 +2288,7 @@ static int vfio_iommu_type1_attach_group(void *iommu_data, * testing if they're on the same bus_type. */ list_for_each_entry(d, &iommu->domain_list, next) { - if (d->domain->ops == domain->domain->ops && - d->enforce_cache_coherency == - domain->enforce_cache_coherency) { + if (d->domain->ops == domain->domain->ops) { iommu_detach_group(domain->domain, group->iommu_group); if (!iommu_attach_group(d->domain, group->iommu_group)) { -- 2.17.1