Re: [Xen-devel] IOMMU initialization failure when using linux-as-bootloader

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 29.08.16 at 14:00, <s.munaut@xxxxxxxxxxxxxxxxxxxx> wrote:
>> I think this is reasonable, despite it certainly being unexpected for
>> the BIOS to turn such on when not putting the system into x2APIC
>> mode.
> 
> It seems that linux doesn't use x2APIC because the BIOS explicitly
> "opts out" of it :
> 
> "DMAR-IR: x2apic is disabled because BIOS sets x2apic opt out bit."

Note the "not" in my earlier reply. Or otherwise I don't understand
what you're trying to tell me.

>> Considering you're not talking about a BIOS here, may I
>> nevertheless ask why it's not instead Linux/kexec which get fixed
>> to not leave these features enabled before handing off control?
> 
> I think both should be done.
> 
> Xen should tolerate that case and not assume that if it's enabled it's
> been enabled by Xen.

That's debatable, but as said, your draft patch looks reasonable.
The problem here is that (as so many things in this area) what
state a kernel is allowed to expect the system to be in when
handled control for the first time.

> Especially since that "pre-cleanup" already exists in one path and not
> the other.

On that other path it's a requirement, since firmware can indeed
be expected to turn on interrupt remapping when coming up in
x2APIC mode. Plus ...

> But the kernel should definitely clean up as much as possible after
> itself before reboot.
> I've actually been looking into that this morning but didn't come up
> with a solution yet.
> 
> The Xen case was easy because I just had to copy the missing calls
> from one codepath to the other, for the linux path I'm still looking
> how & where to do that. If I'm still no closer to an answer by
> tomorrow, I'll ask on the kexec mailing list if anyone has any
> suggestion.

... this makes clear that you're acting phenomenologically. For us
to be safe in this regard, however, it would require that we put
back into a clean state _everything_ that the firmware (or Linux in
your present case) may have (pre-)initialized.

Jan


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel