diff mbox series

[v3,2/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency

Message ID 20220623200029.26007-3-nicolinc@nvidia.com
State Changes Requested
Headers show
Series Simplify vfio_iommu_type1 attach/detach routine | expand

Commit Message

Nicolin Chen June 23, 2022, 8 p.m. UTC 
From: Jason Gunthorpe <jgg@nvidia.com>

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 <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
---
 drivers/vfio/vfio_iommu_type1.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

Comments

Baolu Lu June 24, 2022, 1:50 a.m. UTC | #1 
On 2022/6/24 04:00, Nicolin Chen wrote:
> From: Jason Gunthorpe <jgg@nvidia.com>
> 
> 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 <jgg@nvidia.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
> ---
>   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 <baolu.lu@linux.intel.com>

Best regards,
baolu
diff mbox series

Patch

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)) {