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Date:   Thu, 24 Sep 2020 11:08:57 +0200
From:   Joerg Roedel <joro@8bytes.org>
To:     Baoquan He <bhe@redhat.com>
Cc:     ahuang12@lenovo.com, iommu@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] Revert "iommu/amd: Treat per-device exclusion ranges as
 r/w unity-mapped regions"
Message-ID: <20200924090857.GG27174@8bytes.org>
References: <20200923022655.750-1-bhe@redhat.com>
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On Wed, Sep 23, 2020 at 10:26:55AM +0800, Baoquan He wrote:
> A regression failure of kdump kernel boot was reported on a HPE system.
> Bisect points at commit 387caf0b759ac43 ("iommu/amd: Treat per-device
> exclusion ranges as r/w unity-mapped regions") as criminal. Reverting it
> fix the failure.
> 
> With the commit, kdump kernel will always print below error message, then
> naturally AMD iommu can't function normally during kdump kernel bootup.
> 
>   ~~~~~~~~~
>   AMD-Vi: [Firmware Bug]: IVRS invalid checksum
> 
> Why commit 387caf0b759ac43 causing it haven't been made clear.

I think this should be debugged further, in future IOMMUs the exclusion
range feature will not be available anymore (mmio-fields get re-used for
SNP). So starting to use them again is not going to work.

Regards,

	Joerg