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

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Date:   Wed, 15 Jun 2022 17:03:01 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     <joro@...tes.org>, <will@...nel.org>, <marcan@...can.st>,
        <sven@...npeter.dev>, <robin.murphy@....com>,
        <robdclark@...il.com>, <baolu.lu@...ux.intel.com>,
        <matthias.bgg@...il.com>, <orsonzhai@...il.com>,
        <baolin.wang7@...il.com>, <zhang.lyra@...il.com>,
        <jean-philippe@...aro.org>, <alex.williamson@...hat.com>,
        <jgg@...dia.com>, <kevin.tian@...el.com>
CC:     <suravee.suthikulpanit@....com>, <alyssa@...enzweig.io>,
        <dwmw2@...radead.org>, <yong.wu@...iatek.com>,
        <mjrosato@...ux.ibm.com>, <gerald.schaefer@...ux.ibm.com>,
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        <isaacm@...eaurora.org>, <yangyingliang@...wei.com>,
        <jordan@...micpenguin.net>, <iommu@...ts.linux-foundation.org>,
        <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-arm-msm@...r.kernel.org>,
        <linux-mediatek@...ts.infradead.org>, <linux-s390@...r.kernel.org>,
        <linux-tegra@...r.kernel.org>,
        <virtualization@...ts.linux-foundation.org>, <kvm@...r.kernel.org>
Subject: [PATCH v2 2/5] vfio/iommu_type1: Prefer to reuse domains vs match enforced cache coherency

From: Jason Gunthorpe <jgg@...dia.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.

Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
Signed-off-by: Nicolin Chen <nicolinc@...dia.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)) {
-- 
2.17.1