On Wed, Jul 22, 2020 at 12:34:57PM +0000, Sironi, Filippo wrote:
> On Wed, 2020-07-22 at 14:19 +0200, j...@8bytes.org wrote:
> I wouldn't be surprised if a PCIe device raises a PCIe SERR if it is
> asked to do DMA beyond its abilities.
Yeah, but that would also make it impossible to safely assign the device
to any untrusted entity, like a guest of user-space driver.
> I think that this discussion is orthogonal wrt the spirit of the
> patches. They are actually adding a few bits to the AMD IOMMU driver
> (and propagating them to the right places) to implement a portion of the
> specification that wasn't implemented, i.e., relying on the IVRS table.
> These patches are valuable independently on the content of the IVRS
> table, be it 32, 48, or 64 bits.
You are right from a technical point of view, and the patches are as
well. The problem I see is that there are a lot of systems out there
with an AMD IOMMU and possibly broken ACPI tables. And if the driver
starts checking this field now it is possible that it breaks formerly
working setups.
So doing this needs a strong reason, like upcoming hardware that has
lower limits in the supported address space size than before. The
use-case you have described is not a strong enough reason to take the
risk.
Regards,
Joerg
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