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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 18.08.2021 13:32, Andrew Cooper wrote:
> 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.

That's your view, and I accept it as a reasonable one. I don't accept
it as being the only reasonable one though, and hence I object to you
tagging other views (here just like in various cases elsewhere) as
"buggy" (or sometimes worse).

>> 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.

Should behave the same - yes. Previous generations without interrupt
remapping also shouldn't allow pass-through by default, i.e. require
admin consent to run guests in this less secure mode (except, perhaps,
for devices without interrupts, albeit I'm unaware of ways to tell).

Jan