linux-kernel - Re: [PATCH] iommu/amd: Enable swiotlb if any device supports iommu v2 and uses identity mapping

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Message-ID: <CAAd53p40RcG0oeYr9QAKMjYRtyq7he=d_b_a39n4Rt5JSVScRQ@mail.gmail.com>
Date:   Thu, 8 Jul 2021 21:57:00 +0800
From:   Kai-Heng Feng <kai.heng.feng@...onical.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Joerg Roedel <joro@...tes.org>, will@...nel.org,
        "open list:AMD IOMMU (AMD-VI)" <iommu@...ts.linux-foundation.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] iommu/amd: Enable swiotlb if any device supports iommu v2
 and uses identity mapping

On Thu, Jul 8, 2021 at 6:18 PM Robin Murphy <robin.murphy@....com> wrote:
>
> On 2021-07-08 10:28, Joerg Roedel wrote:
> > On Thu, Jul 08, 2021 at 03:42:32PM +0800, Kai-Heng Feng wrote:
> >> @@ -344,6 +344,9 @@ static int iommu_init_device(struct device *dev)
> >>
> >>              iommu = amd_iommu_rlookup_table[dev_data->devid];
> >>              dev_data->iommu_v2 = iommu->is_iommu_v2;
> >> +
> >> +            if (dev_data->iommu_v2)
> >> +                    swiotlb = 1;
> >
> > This looks like the big hammer, as it will affect all other systems
> > where the AMD GPUs are in their own group.
> >
> > What is needed here is an explicit check whether a non-iommu-v2 device
> > is direct-mapped because it shares a group with the GPU, and only enable
> > swiotlb in this case.
>
> Right, it's basically about whether any DMA-limited device might at any
> time end up in an IOMMU_DOMAIN_IDENTITY domain. And given the
> possibility of device hotplug and the user being silly with the sysfs
> interface, I don't think we can categorically determine that at boot time.
>
> Also note that Intel systems are likely to be similarly affected (in
> fact intel-iommu doesn't even have the iommu_default_passthough() check
> so it's probably even easier to blow up).

swiotlb is enabled by pci_swiotlb_detect_4gb() and intel-iommu doesn't
disable it.

I wonder if we can take the same approach in amd-iommu?

Kai-Heng

>
> Robin.