Re: [Xen-devel] [PATCH 2 of 2] vpmu: Add a vpmu cpuid function

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-01-20 at 12:40 +0000, Dietmar Hahn wrote:
> Hi Ian,
> 
> Am Freitag 20 Januar 2012, 10:54:44 schrieb Keir Fraser:
> > On 20/01/2012 10:49, "Dietmar Hahn" <dietmar.hahn@xxxxxxxxxxxxxx> wrote:
> > 
> > >> Our CPUID configuration is done per-domain, and from
> > >> tools/libxc/xc_cpuid_x86.c. CPUID adjustments implemented within the
> > >> hypervisor are generally not acceptable without very good reason.
> > > 
> > > Then a way is needed to have access to the opt_vpmu_enabled variable 
> > > within
> > > the
> > > hypervisor from the tools to decide the enabling of the flag (is there 
> > > such a
> > > way?) or the mechanism with the boot variable must be changed.
> > > The opt_vpmu_enabled boot variable was introduced because of a PMU 
> > > problem in
> > > the Nehalem cpus leading sometimes to hypervisor crashes. But with the 
> > > done
> > > quirk we never had a crash anymore.
> > > So maybe we can always switch on the vpmu stuff in the hypervisor and add 
> > > a
> > > flag in the domain configuration when somebody wants to do some 
> > > performance
> > > tests?
> > 
> > Yes!
> > 
> > It's obviously an option of fairly narrow interest. If someone tries to
> > enable the per-domain option on a CPU which has problems, you can fail the
> > domain creation, or print a warning in the hypervisor log, or whatever. Any
> > sensible option in that respect is fine by me!
> 
> What is the best solution for this?
> A domain specific configuration option is needed (vpmu?) which is usable in
> libxc/xc_cpuid_x86.c to select/deselect special vpmu bits in the cpuid 
> command.
> Can you point me to an proper example?

Can't this already be done via the cpuid domain option given the correct
runes? Maybe with an addition to the table in libxl_cpuid_parse_config?

Ian.



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