Re: [PATCH] xen/x86: Make XEN_DOMCTL_get_vcpu_msrs more configurable

[Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>]

On Tue, Oct 25, 2022 at 4:13 AM Roger Pau Monné <roger.pau@xxxxxxxxxx> wrote:
>
> On Mon, Oct 24, 2022 at 12:58:54PM -0400, Tamas K Lengyel wrote:
> > Currently the XEN_DOMCTL_get_vcpu_msrs is only capable of gathering a handful
> > of predetermined vcpu MSRs. In our use-case gathering the vPMU MSRs by an
> > external privileged tool is necessary, thus we extend the domctl to allow for
> > querying for any guest MSRs. To remain compatible with the existing setup if
> > no specific MSR is requested via the domctl the default list is returned.
>
> I'm afraid I would benefit from some extra description about why you
> need to introduce a separate hook instead of using the existing
> do_rdmsr hook in arch_vpmu_ops (which is already hooked into
> guest_rdmsr()).
>
> Are the MSRs you are trying to fetch not accessible for the guest
> itself to read?

No, the reason we need this different hook is because do_rdmsr assumes the guest is reading the MSRs that are currently loaded. For external tools where v != current the vpmu context needs to be saved by pausing the vcpu first and then the MSR content returned from the saved context.

 Tamas