Re: [Xen-devel] [PATCH] VT-d/DMAR: accept DRHD with non-discoverable PCI devices

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 21/03/2019 13:17, Andrew Cooper wrote:
> On 20/03/2019 20:22, Igor Druzhinin wrote:
>> Since commit dcf41790 ("x86/mmcfg/drhd: Move acpi_mmcfg_init() call
>> before calling acpi_parse_dmar()") PCI segment 0 is now known early
>> which made the sanity check on DRHD definition structure to work.
>> This, in turn, caused a regression on some machines (in particular,
>> HP PowerEdge R740 with I/O AT DMA disabled) where IOMMU was explicitly
>> disabled due to some internal PCI devices being non-discoverable but
>> present in DMAR.
>>
>> While this is indeed a BIOS mistake it seems to be not that critical
>> to disable the whole IOMMU. Instead, extend the scope of
>> "workaround_bios_bug" option and make it enabled by default. This is
>> consistent with our documentation and actually what a user might expect
>> from an option with that name. It also doesn't seem safe to simply ignore
>> DRHD without initialization so remove this case. But leave the original
>> DMAR check in place to still allow error reporting.
>>
>> Signed-off-by: Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>
> 
> This is code which was previously dead.  The behaviour of ignoring an
> IOMMU because there is unreachable device behind it is awful and
> shouldn't exist, but we should at least leave a trace of it in the logs.
> 
> TBH, I'd prefer to delete the `workaround_bios_bug` option entirely, and
> just print out the bad entries.  Nothing the option does is worthy of
> shutting the IOMMU down.

The option left there to provide an easy fallback to the originally
intended behavior in case someone has a strong opinion that it might be
useful. Other than that, I'm ok with removing it.

Igor

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