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

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Date:   Fri, 22 Sep 2023 23:41:17 +0900
From:   Hector Martin <marcan@...can.st>
To:     Robin Murphy <robin.murphy@....com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        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
Subject: Re: [PATCH REGRESSION] iommu: Only allocate FQ domains for IOMMUs
 that support them

On 22/09/2023 23.21, Robin Murphy wrote:
> 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.

Sure, but it sounded like if there's a capability for this the core
should probably use it and not expose the type at all to drivers that
can't support it :)

If you think defaulting to that branch in DART is correctly future-proof
I can make that change. It's not the only driver checking the domain
type in attach_dev(), but it might be the only one enumerating all the
options instead of checking for specific cases only (e.g. intel checks
for IOMMU_DOMAIN_IDENTITY).

- Hector