[PATCH v3 2/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency
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 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 c13b9290e357..f4e3b423a453 100644 --- a/drivers/vfio/vfio_iommu_type1.c +++ b/drivers/vfio/vfio_iommu_type1.c @@ -2285,9 +2285,7 @@ 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
Re: [PATCH v3 2/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency
On 2022/6/24 04:00, Nicolin Chen wrote: 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 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 c13b9290e357..f4e3b423a453 100644 --- a/drivers/vfio/vfio_iommu_type1.c +++ b/drivers/vfio/vfio_iommu_type1.c @@ -2285,9 +2285,7 @@ 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)) { Reviewed-by: Lu Baolu Best regards, baolu ___ iommu mailing list iommu@lists.linux-foundation.org https://lists.linuxfoundation.org/mailman/listinfo/iommu