Re: [PATCH v2 0/2] kvm: x86: Emulate MSR_PLATFORM_INFO

[Gleb Natapov <gleb@redhat.com>]

On Tue, Jun 18, 2013 at 11:29:27AM -0400, Bandan Das wrote:
> Gleb Natapov <gleb@redhat.com> writes:
> 
> > On Tue, Jun 18, 2013 at 04:05:08PM +0200, Paolo Bonzini wrote:
> >> Il 05/06/2013 10:42, Gleb Natapov ha scritto:
> >> >> > These patches add an emulated MSR_PLATFORM_INFO that kvm guests
> >> >> > can read as described in section 14.3.2.4 of the Intel SDM. 
> >> >> > The relevant changes and details are in [2/2]; [1/2] makes vendor_intel
> >> >> > generic. There are atleat two known applications that fail to run because
> >> >> > of this MSR missing - Sandra and vTune.
> >> > So I really want Intel opinion on this. Right now it is impossible to
> >> > implement the MSR correctly in the face of migration (may be with tsc
> >> > scaling it will be possible) and while it is unimplemented if application
> >> > tries to use it it fails, but if we will implement it application will
> >> > just produce incorrect result without any means for user to detect it.
> >> 
> >> Jun, ping?  (Perhaps Gleb you want to ask a more specific question though).
> >> 
> >> I don't think this is much different from any other RDTSC usage in
> >> applications (they will typically do their calibration manually, and do
> >> it just once).  I'm applying it to queue.
> >> 
> > And we do not support application that uses RDTSC directly! If we could
> > catch those it would be good from support point of view, so the way
> > MSR_PLATFORM_INFO behaves now it better then proposed alternative.
> 
> If support is the issue, can't we have a flag that disables this by
> default and users who want to take the plunge (and be responsible
> for the consequences) can enable it to read platform_info ?
> 
We have it already :) ignore_msrs. If it is set unimplemented MSRs will
not inject #GP, but return zero value instead. Zero it as incorrect as
anything else in the case of migration.

--
			Gleb.