Re: [PATCH v2 2/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency

[Alex Williamson <alex.williamson@redhat.com>]

On Wed, 15 Jun 2022 17:03:01 -0700
Nicolin Chen <nicolinc@nvidia.com> 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 those devices are
> attached to a single domain or multiple domains.
> 
> So, there is no value in 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 since KVM won't be able to take advantage of it.
> This just wastes domain memory.
> 
> 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.

The 2nd paragraph above is quite misleading in this respect.  I think
it would be more accurate to explain that the benefit to using separate
domains was that 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.  We can then go on
to discuss that in practice 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.  Thanks,

Alex

> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.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)) {