Re: [Xen-devel] [PATCH for-4.13] AMD/IOMMU: honour IR setting while pre-filling DTEs

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 26/11/2019 14:14, Jan Beulich wrote:
> On 26.11.2019 13:25, Andrew Cooper wrote:
>> On 26/11/2019 08:42, Jan Beulich wrote:
>>> On 25.11.2019 22:05, Igor Druzhinin wrote:
>>>> --- a/xen/drivers/passthrough/amd/iommu_init.c
>>>> +++ b/xen/drivers/passthrough/amd/iommu_init.c
>>>> @@ -1279,7 +1279,7 @@ static int __init amd_iommu_setup_device_table(
>>>>          for ( bdf = 0, size /= sizeof(*dt); bdf < size; ++bdf )
>>>>              dt[bdf] = (struct amd_iommu_dte){
>>>>                            .v = true,
>>>> -                          .iv = true,
>>>> +                          .iv = iommu_intremap,
>>> This was very intentionally "true", and ignoring "iommu_intremap":
>>
>> Deliberate or not, it is a regression from 4.12.
> 
> I accept it's a regression (which wants fixing), but I don't think
> this is the way to address is. I could be convinced by good
> arguments, though.

Do you have any suggestions how to address that?

>> Booting with iommu=no-intremap is a common debugging technique, and that
>> means no interrupt remapping anywhere in the system, even for
>> supposedly-unused DTEs.
> 
> Whether IV=1 or IV=0, there's no interrupt _remapping_ with this
> option specified. There's some interrupt _blocking_, yes. It's
> not immediately clear to me whether this is a good or a bad thing.

From user point of view, if I supply "iommu=no-intremap" I'm not
expecting any interrupts in the system to be blocked either. And
as Andrew said we frequently use this option for debugging which
means we expect this functionality to be off completely.

Igor

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