Re: [PATCH 7/7] x86/viridian: implement the crash MSRs

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@suse.com]
> Sent: 20 March 2017 13:29
> To: Paul Durrant <Paul.Durrant@citrix.com>
> Cc: Andrew Cooper <Andrew.Cooper3@citrix.com>; Ian Jackson
> <Ian.Jackson@citrix.com>; Wei Liu <wei.liu2@citrix.com>; xen-
> devel@lists.xenproject.org
> Subject: RE: [PATCH 7/7] x86/viridian: implement the crash MSRs
> 
> >>> On 20.03.17 at 13:48, <Paul.Durrant@citrix.com> wrote:
> >> From: Jan Beulich [mailto:JBeulich@suse.com]
> >> Sent: 20 March 2017 12:38
> >> >>> On 17.03.17 at 10:57, <paul.durrant@citrix.com> wrote:
> >> > --- a/xen/include/asm-x86/hvm/viridian.h
> >> > +++ b/xen/include/asm-x86/hvm/viridian.h
> >> > @@ -96,6 +96,7 @@ struct viridian_domain
> >> >      union viridian_hypercall_gpa hypercall_gpa;
> >> >      struct viridian_time_ref_count time_ref_count;
> >> >      union viridian_reference_tsc reference_tsc;
> >> > +    uint64_t crash_param[5];
> >>
> >> Are these really per-domain values (normally MSRs are per-vCPU)?
> >
> > I don't think the usage described in section 4.3 really warrants per-vCPU.
> > I've not looked at the exact sequence of events but I'm fairly sure the
> MSRs
> > are written by the Windows crash kernel, which is single threaded.
> 
> Well, what I'm wondering about here is whether it wouldn't be
> possible (and reasonable) to have two vCPU-s crashing at almost
> the same time write independent values. They might then even
> sync with one another to settle on who of them should do the
> final MSR write. But if they're indeed only (meant to be) written
> by the (single threaded) crash kernel (which I find strange), then
> of course these are fine per-domain.
> 

Ok. Given it's not very much data, I'll make them per-vcpu just in case.

> >> And don't they need migrating?
> >
> > I don't think so, given that they are only written just prior to the domain
> > dying anyway.
> 
> Well, that leaves the risk of the host admin to decide moving the
> domain the very moment it is doing its final crash processing.
> Which may of course be an acceptable risk.
> 

I think that's probably an acceptable risk. I won't add them to the migration stream for the moment.

  Paul

> Jan


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