linux-kernel - [PATCH REGRESSION] iommu: Only allocate FQ domains for IOMMUs that support them

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Message-Id: <20230922-iommu-type-regression-v1-1-1ed3825b2c38@marcan.st>
Date:   Fri, 22 Sep 2023 22:40:20 +0900
From:   Hector Martin <marcan@...can.st>
To:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        Jerry Snitselaar <jsnitsel@...hat.com>
Cc:     Joerg Roedel <jroedel@...e.de>, Neal Gompa <neal@...pa.dev>,
        "Justin M. Forbes" <jforbes@...oraproject.org>,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        asahi@...ts.linux.dev, stable@...r.kernel.org,
        regressions@...ts.linux.dev, Hector Martin <marcan@...can.st>
Subject: [PATCH REGRESSION] iommu: Only allocate FQ domains for IOMMUs that
 support them

Commit a4fdd9762272 ("iommu: Use flush queue capability") hid the
IOMMU_DOMAIN_DMA_FQ domain type from domain allocation. A check was
introduced in iommu_dma_init_domain() to fall back if not supported, but
this check runs too late: by that point, devices have been attached to
the IOMMU, and the IOMMU driver might not expect FQ domains at
ops->attach_dev() time.

Ensure that we immediately clamp FQ domains to plain DMA if not
supported by the driver at device attach time, not later.

This regressed apple-dart in v6.5.

Cc: regressions@...ts.linux.dev
Cc: stable@...r.kernel.org
Fixes: a4fdd9762272 ("iommu: Use flush queue capability")
Signed-off-by: Hector Martin <marcan@...can.st>
---
 drivers/iommu/iommu.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
index 3bfc56df4f78..12464eaa8d91 100644
--- a/drivers/iommu/iommu.c
+++ b/drivers/iommu/iommu.c
@@ -2039,6 +2039,15 @@ static int __iommu_attach_device(struct iommu_domain *domain,
 	if (unlikely(domain->ops->attach_dev == NULL))
 		return -ENODEV;
 
+	/*
+	 * Ensure we do not try to attach devices to FQ domains if the
+	 * IOMMU does not support them. We can safely fall back to
+	 * non-FQ.
+	 */
+	if (domain->type == IOMMU_DOMAIN_DMA_FQ &&
+	    !device_iommu_capable(dev, IOMMU_CAP_DEFERRED_FLUSH))
+		domain->type = IOMMU_DOMAIN_DMA;
+
 	ret = domain->ops->attach_dev(domain, dev);
 	if (ret)
 		return ret;

---
base-commit: ce9ecca0238b140b88f43859b211c9fdfd8e5b70
change-id: 20230922-iommu-type-regression-25b4f43df770

Best regards,
-- 
Hector Martin <marcan@...can.st>