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

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Date:   Fri, 22 Sep 2023 11:42:22 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Hector Martin <marcan@...can.st>, Joerg Roedel <joro@...tes.org>,
        Will Deacon <will@...nel.org>,
        Jerry Snitselaar <jsnitsel@...hat.com>,
        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 Fri, Sep 22, 2023 at 03:21:17PM +0100, 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;

Yes, I much prefer this to the original patch please. Drivers should
not be testing DMA_FQ at all.

I already wrote a series to convert DART to domain_alloc_paging() that
fixes this inadvertantly.

Robin's suggestion is good for a temporary -rc fix.

Removing the switch is slightly more robust:

if (domain->type & domain->type & __IOMMU_DOMAIN_PAGING) {
  [..]
  return 0
}

if (domain->type == IOMMU_DOMAIN_BLOCKED) {
  ..
}

return -EOPNOTSUPP;

But not so worthwhile since I deleted all this anyhow...

I'll send out the dart series, it can't go to -rc, so a patch is still needed.

Thanks,
Jason