linux-kernel - Re: [PATCH] iommu/amd: Enforce 4k mapping for certain IOMMU data structures

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Message-ID: <20201117225717.GF524@willie-the-truck>
Date:   Tue, 17 Nov 2020 22:57:17 +0000
From:   Will Deacon <will@...nel.org>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc:     linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        joro@...tes.org, Jon.Grimm@....com, brijesh.singh@....com
Subject: Re: [PATCH] iommu/amd: Enforce 4k mapping for certain IOMMU data
 structures

On Wed, Oct 28, 2020 at 11:18:24PM +0000, Suravee Suthikulpanit wrote:
> AMD IOMMU requires 4k-aligned pages for the event log, the PPR log,
> and the completion wait write-back regions. However, when allocating
> the pages, they could be part of large mapping (e.g. 2M) page.
> This causes #PF due to the SNP RMP hardware enforces the check based
> on the page level for these data structures.

Please could you include an example backtrace here?

> So, fix by calling set_memory_4k() on the allocated pages.

I think I'm missing something here. set_memory_4k() will break the kernel
linear mapping up into page granular mappings, but the IOMMU isn't using
that mapping, right? It's just using the physical address returned by
iommu_virt_to_phys(), so why does it matter?

Just be nice to capture some of this rationale in the log, especially as
I'm not familiar with this device.

> Fixes: commit c69d89aff393 ("iommu/amd: Use 4K page for completion wait write-back semaphore")

I couldn't figure out how that commit could cause this problem. Please can
you explain that to me?

Cheers,

Will