Re: [Xen-devel] Unable to boot Xen 4.8 with iommu=0

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Thu, Feb 16, 2017 at 11:53 PM, Tian, Kevin <kevin.tian@xxxxxxxxx> wrote:
>> From: Tian, Kevin
>> Sent: Friday, February 17, 2017 11:35 AM
>> > >>
>> > >> Or wait - do you have the same issue if you use
>> > >> "iommu=no,no-intremap"? In which case the problem would be
>> > >> that "iommu=no" should clear more than just "iommu_enable", or
>> > >> code checking iommu_intremap early (before iommu_setup()
>> > >> manages to clear it in the case here) would need to made look at
>> > >> both variables. Oddly enough acpi_parse_dmar() only bails if
>> > >> both variables are clear, which suggests to me that
>> > >> iommu_enable is intended to have two different meanings in
>> > >> different contexts (master flag vs. controlling just DMA
>> > >> remapping). Kevin, Feng - any thoughts here?
>> > >
>> > > iommu=no,no-intremap boots fine with "(XEN) Using APIC driver default"
>> >
>> > Thanks for confirming.
>> >
>> > Kevin, Feng, we now depend on your input regarding the intentions
>> > with the two variables.
>> >
>>
>> Feng just left Intel. Let me take a look at code to understand the
>> rationale behind.
>>
>
> Jan, looks it's caused by your change back to 2012:
>
> commit 7a8f6d0607a38c64506b4e8b473d955bf8e2a71f
> Author: Jan Beulich <jbeulich@xxxxxxxx>
> Date:   Fri Nov 2 17:15:30 2012 +0100
>
> Before that iommu_enable was the master flag consistently. I'm still
> trying to understand the background and you may help elaborate if
> still something in your memory.
>
> I agree we should stick to one meaning after clearing up above issue.
>
> Given this commit is pretty old, I'm also curious why it's only reported
> on 4.8. Tamas, did you succeed with iommu=0 pre 4.8, or 4.8 happens
> to be the one upon which you first tried iommu=0 on a platform supporting
> interrupt remapping?

It just happens to be the one I tried. VT-d was spamming my console
with faults like this:

(XEN) [VT-D]DMAR:[DMA Write] Request device [0000:00:02.0] fault addr
277e28000, iommu reg = ffff82c000201000
(XEN) [VT-D]DMAR: reason 05 - PTE Write access is not set

So I figured I can just turn it off to clean up my console. I still
don't know what these VT-d faults are about..

Tamas

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