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Re: [PATCH] xen/x86: Make XEN_DOMCTL_get_vcpu_msrs more configurable
- To: Tamas K Lengyel <tamas.k.lengyel@xxxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Wed, 26 Oct 2022 12:02:54 +0200
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- Cc: Tamas K Lengyel <tamas.lengyel@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, Wei Liu <wl@xxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Jun Nakajima <jun.nakajima@xxxxxxxxx>, Kevin Tian <kevin.tian@xxxxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On Tue, Oct 25, 2022 at 01:48:36PM -0400, Tamas K Lengyel wrote:
> 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.
Hm, I see.
We need to dump the CPU MSR contents into the structure so they can be
read from a different pCPU differently than the currently running one.
It would be nice if this could all be somehow wired into
guest_rdmsr(), but the function executing a vcpu_pause() as part of
it's operations would be quite weird, also it having a vcpu parameter
is kind of misleading, as under some circumstances it will perform a
rdmsr and that's likely only correct when v == current.
I guess I will ask for others opinion, but having that specific vPMU
function call in XEN_DOMCTL_get_vcpu_msrs on the side of guest_rdmsr()
seems like a layering violation, as it should all be contained in
guest_rdmsr().
Thanks, Roger.
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