Re: [PATCH v11 1/9] x86: add generic MSR access hypercall

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Mon, Jun 23, 2014 at 07:29:41AM +0000, Xu, Dongxiao wrote:
> > -----Original Message-----
> > From: Jan Beulich [mailto:JBeulich@suse.com]
> > Sent: Monday, June 23, 2014 2:45 PM
> > To: Xu, Dongxiao
> > Cc: andrew.cooper3@citrix.com; Ian.Campbell@citrix.com;
> > George.Dunlap@eu.citrix.com; Ian.Jackson@eu.citrix.com;
> > stefano.stabellini@eu.citrix.com; xen-devel@lists.xen.org;
> > konrad.wilk@oracle.com; dgdegra@tycho.nsa.gov; keir@xen.org
> > Subject: RE: [PATCH v11 1/9] x86: add generic MSR access hypercall
> > 
> > >>> On 23.06.14 at 08:27, <dongxiao.xu@intel.com> wrote:
> > >>  -----Original Message-----
> > >> From: Jan Beulich [mailto:JBeulich@suse.com]
> > >> Sent: Friday, June 20, 2014 11:01 PM
> > >> To: Xu, Dongxiao
> > >> Cc: andrew.cooper3@citrix.com; Ian.Campbell@citrix.com;
> > >> George.Dunlap@eu.citrix.com; Ian.Jackson@eu.citrix.com;
> > >> stefano.stabellini@eu.citrix.com; xen-devel@lists.xen.org;
> > >> konrad.wilk@oracle.com; dgdegra@tycho.nsa.gov; keir@xen.org
> > >> Subject: Re: [PATCH v11 1/9] x86: add generic MSR access hypercall
> > >>
> > >> >>> On 20.06.14 at 16:31, <dongxiao.xu@intel.com> wrote:
> > >> > Add a generic MSR access hypercall for tool stack or other components to
> > >> > read or write certain system MSRs.
> > >>
> > >> If you want this to be usable by components other than the tool stack
> > >> then this can't be a sysctl (would e.g. need to be a platform-op then).
> > >
> > > Per my understanding, the platform-op related hypercalls are used by Dom0
> > > kernel.
> > > While in this CQM scenario, we want libvirt/libxl/libxc to call such
> > > hypercall to access the MSR, however I didn't see any usage of platform_op in
> > > libxl/libxc side.
> > >
> > > Could you explain a bit more for it?
> > 
> > Platform ops are for use by Dom0, yes, but note the explicit omission
> > of "kernel" in my reply. The fact that libxc currently doesn't use any
> > doesn't mean it mustn't do so. The only question here that matter is
> > what audience we see for the new functionality: If this is to be a
> > purely tools interface, then a sysctl is fine. If the kernel is intended to
> > also be able to use it (as I suggested), something other than a sysctl
> > needs to be used (and I was merely _hinting_ at platform-op).
> 
> I got such concept in the file descriptor, and that's why I was thinking of platform-op is for dom0-kernel only...
> Looking through all the current hypercalls, sysctl and platform-op are the most appropriate ones.

Keep in mind that the sysctl one has a version field that is tied in with
the toolstack. So if you want this hypercall to work across different
versions (like the kernel has to), then you do _not_ want to use
sysctl.