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Re: [Xen-devel] [PATCH 3/3] x86/AMD-Vi: Fix IVRS HPET special->handle override

On 9/23/2013 1:52 AM, Jan Beulich wrote:
On 20.09.13 at 23:38, Suravee Suthikulanit <suravee.suthikulpanit@xxxxxxx>
On 9/17/2013 10:30 AM, Jan Beulich wrote:
This (unconditional) assignment is what the earlier logic attempted
to avoid: We must not blindly set this (and in particular not blindly
overwrite a previously set valid value), and in order to do so we
need to know whether to trust devid or handle. I'm therefore going
to apply only the first hunk - being a clear and obvious bug fix - for
the time being.

But since we only allow one HPET in the system, and if users want to
override the one that is in the IVRS (the buggy one)
we should allow this, right?  Otherwise, if the special->handle doesn't
match, it would end up causing this logic to complain
about multiple HPETs.
We must not allow multiple HPET entries in the ACPI tables to
confuse us and store a bad IOMMU pointer.
I understand this part and agree.  Currently, the patch does not allow 
multiple HPET in the system.
If user specifies the ivrs_hpet, that will be the one that get used, and 
ignore the one that
is in the IVRS.  Although, it will be pointing to the IOMMU which lists 
HPET in the IVRS.

However, if IVRS is listing multiple HPETs in different IOMMUs, then it 
will just default to the first IOMMU.
I don't see this case happening though since HPET is in the Southbridge 
which only has one in the system.

Am I missing any thing?
Yes - there's no guarantee that (especially in a multi-node
system) there's just one HPET. Nor do the ACPI tables have
any indication there this would always be the case. Even if
_all_ current systems only have a single HPET (which I don't
think you can guarantee), we shouldn't code in a latent bug
like this.

I am a bit confused on what you want to do.  I believe all the systems at this point
should have only one HPET.  That's why the code only has one data structure for
initialize one HPET.  Are you expecting that certain systems could have more than
one HPETs?

Alternatively, I can try matching on the "SBDF" assuming that IVRS doesn't screw up
the SBDF part of the HPET special entry.

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