Re: [Xen-devel] [PATCH v5 6/6] amd/iommu: skip bridge devices when updating IOMMU page tables

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Nov 21, 2018 at 02:21:36AM -0700, Jan Beulich wrote:
> >>> On 21.11.18 at 00:26, <Brian.Woods@xxxxxxx> wrote:
> > The original commit 0af438757d455f8eb6b5a6ae9a990ae245f230fd
> > 
> > The commit that adds is_hardware_domain (and rearrange things)
> > 7c275549f46c5c46611592f7107c1345e93ed457
> > 
> > The orginal commit used the function like
> > setup_dom0_pci_devices(d, amd_iommu_setup_dom0_device);
> > which was because IOMMU needed to skip the host bridge devices on dom0.
> > 
> > So I assume you added the is_hardware_domain because it only needed to
> > be done on dom0.  I'm not familiar with the IOMMU/PCI history wrt to
> > what it mapped/passed through so.
> 
> Well, I added it presumably to retain original semantics. I still
> think that the extra check would better be dropped there, not
> the least to also cover the case of devices eventually getting
> assigned to dom_xen.
> 
> Looking at this another time I find some other questionable
> aspect though (both to pre-existing code and to the change
> made here): "host bridge" != "bridge". The title here as much
> as the comment next to the original piece of code both
> suggest the wider general category is meant, but the code
> cloned checks for host bridges only. In
> amd_iommu_add_device() the check is used solely to emit a
> less scary log message, but the change here goes beyond
> that.

The check in amd_iommu_add_device allows host bridge devices to be
assigned to the hardware domain without returning an error since they
are not behind an IOMMU. Note that even if amd_iommu_add_device
returned an error the device would still be added to the hardware
domain since the error is eaten by setup_one_hwdom_device and the
device is assigned to the hardware domain regardless.

So either all devices that are not behind an IOMMU are not assigned to
the hardware domain, or update_paging_mode needs this workaround in
order to be able to handle IOMMU page table expansion for the hardware
domain.

Thanks, Roger.

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