Re: [Xen-devel] [PATCH 2/4] iommu: generalize iommu_inclusive_mapping

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 01.08.18 at 10:32, <andrew.cooper3@xxxxxxxxxx> wrote:
> On 01/08/2018 09:20, Jan Beulich wrote:
>>>>> On 31.07.18 at 17:33, <roger.pau@xxxxxxxxxx> wrote:
>>> On Tue, Jul 31, 2018 at 08:39:22AM -0600, Jan Beulich wrote:
>>>>>>> On 27.07.18 at 17:31, <roger.pau@xxxxxxxxxx> wrote:
>>>>> Introduce a new iommu=inclusive generic option that supersedes
>>>>> iommu_inclusive_mapping. This should be a non-functional change on
>>>>> Intel hardware, while AMD hardware will gain the same functionality of
>>>>> mapping almost everything below the 4GB boundary.
>>>> So first of all - what's the motivation behind this change? So far we
>>>> had no need for hacks line the VT-d side one on AMD. I don't think
>>>> this should be widened without there being indication of a problem
>>>> with non-niche AMD systems.
>>> OK, I can leave the default on for Intel and off for everything else,
>>> but I will introduce the generic dom0-iommu= option anyway.
>> Hmm, I've always been wishing we'd change to a default of off for
>> VT-d as well - imo we shouldn't by default assume broken firmware.
>> Kevin?
>>
>> As to AMD - you still don't really say why this would be needed
>> there. I'm not in favor of workarounds when there's nothing to
>> work around. IOW - if the logic isn't needed on AMD, do we need
>> this code movement in the first place?
> 
> This is sufficiently broken on AMD as well.  Take a look at all the
> IVRS/IVHT reports we've had on xen-devel.

We've had IVRS issues, yes, and we now have command line options
to deal with them. That's entirely unrelated to memory ranges though.

> Furthermore, on by default is the only reasonable option.  Dom0 is
> specifically given cpu mappings of the reserved regions, therefore it
> should have matching IOMMU mappings.

I don't think I can uniformly agree with this: There are reserved
regions which specifically would better not have IOMMU mappings:
LAPIC, IO-APIC, MMCFG just to name things I can immediately
think of.

Jan



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