Re: [PATCH REGRESSION] iommu: Only allocate FQ domains for IOMMUs that support them

[Robin Murphy <robin.murphy@arm.com>]

On 22/09/2023 2:40 pm, Hector Martin wrote:
> 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.

Apologies, I missed that apple-dart was doing something unusual here. 
However, could we just fix that directly instead?

diff --git a/drivers/iommu/apple-dart.c b/drivers/iommu/apple-dart.c
index 2082081402d3..0b8927508427 100644
--- a/drivers/iommu/apple-dart.c
+++ b/drivers/iommu/apple-dart.c
@@ -671,8 +671,7 @@ static int apple_dart_attach_dev(struct iommu_domain 
*domain,
  		return ret;

  	switch (domain->type) {
-	case IOMMU_DOMAIN_DMA:
-	case IOMMU_DOMAIN_UNMANAGED:
+	default:
  		ret = apple_dart_domain_add_streams(dart_domain, cfg);
  		if (ret)
  			return ret;


That's pretty much where we're headed with the domain_alloc_paging 
redesign anyway - at the driver level, operations on a paging domain 
should not need to know about the higher-level usage intent of that 
domain. Ideally, blocking and identity domains should have their own 
distinct ops now as well, but that might be a bit too big a change for 
an immediate fix here.

Thanks,
Robin.

> 
> Cc: regressions@lists.linux.dev
> Cc: stable@vger.kernel.org
> Fixes: a4fdd9762272 ("iommu: Use flush queue capability")
> Signed-off-by: Hector Martin <marcan@marcan.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,