[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH] x86/HVM: support IOMMU-related Viridian CPUID bits

  • To: Jan Beulich <JBeulich@xxxxxxxx>
  • From: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
  • Date: Fri, 1 Aug 2014 15:31:54 +0000
  • Accept-language: en-GB, en-US
  • Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>, "Keir \(Xen.org\)" <keir@xxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Fri, 01 Aug 2014 15:32:00 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>
  • Thread-index: AQHPrY9KGIkeeZPFoUCL92Lj7FjrbZu7xDhQ///laACAACeC0P//4/wAgAAh1KD//+RkgAAES3CA
  • Thread-topic: [PATCH] x86/HVM: support IOMMU-related Viridian CPUID bits

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: 01 August 2014 16:20
> To: Paul Durrant
> Cc: Andrew Cooper; xen-devel; Keir (Xen.org)
> Subject: RE: [PATCH] x86/HVM: support IOMMU-related Viridian CPUID bits
> >>> On 01.08.14 at 17:01, <Paul.Durrant@xxxxxxxxxx> wrote:
> > Maybe. The spec. certainly implies that no Xen guest is ever likely to do
> > anything useful with them though. I'm also concerned about these bits
> > suddenly becoming visible across a migrate.
> Or disappear. But that's a problem with CPUID4A_MSR_BASED_APIC,
> suggest we don't report any CPUID bits depending on some condition
> which may change across migration, until proper validation of feature
> parity of both hosts is in place?

It's also a problem with the TSC/APIC frequency features too - a migration from 
one version of Xen to another can make them magically appear.
For 4A bits, I think we should make sure stuff shouldn't change after boot. For 
6A bits, having re-read the comment in the spec. it sounds like it may be ok:

"0x40000006: Implementation hardware features. Indicates which 
hardware-specific features have been detected and are currently in use by the 

There's nothing to say they can't change, but then there's nothing to say they 

I'm working on a patch to add reference TSC page support at the moment, which 
is dependent on invariant TSC, and the spec. does have provision for that 
feature disappearing on migrate... so maybe we are ok.


Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.