Re: [PATCH] VT-d: Tylersburg errata apply to further steppings

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/08/2021 12:13, Jan Beulich wrote:
> While for 5500 and 5520 chipsets only B3 and C2 are mentioned in the
> spec update, X58's also mentions B2, and searching the internet suggests
> systems with this stepping are actually in use. Even worse, for X58
> erratum #69 is marked applicable even to C2. Split the check to cover
> all applicable steppings and to also report applicable errata numbers in
> the log message. The splitting requires using the DMI port instead of
> the System Management Registers device, but that's then in line (also
> revision checking wise) with the spec updates.
>
> Fixes: 6890cebc6a98 ("VT-d: deal with 5500/5520/X58 errata")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> As to disabling just interrupt remapping (as the initial version of the
> original patch did) vs disabling the IOMMU as a whole: Using a less
> heavy workaround would of course be desirable, but then we need to
> ensure not to misguide the tool stack about the state of the system.

This reasoning is buggy.

This errata is very specifically to do with interrupt remapping only. 
Disabling the whole IOMMU in response is inappropriate.

> It uses the PHYSCAP_directio sysctl output to determine whether PCI pass-
> through can be made use of, yet that flag is driven by "iommu_enabled"
> alone, without regard to the setting of "iommu_intremap".

The fact that range of hardware, including Tylersburg, don't have
interrupt remapping, and noone plumbed this nicely to the toolstack is
suboptimal.

But it is wholly inappropriate to punish users with Tylersburg hardware
because you don't like the fact that the toolstack can't see when
interrupt remapping is off.  The two issues are entirely orthogonal.

Tylersburg (taking this erratum into account) works just as well as and
securely as several previous generations of hardware, and should behave
the same.

~Andrew