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Re: [Xen-devel] [PATCH] x86/vpmu: Add get/put_vpmu() and VPMU_ENABLED

On 02/17/2017 10:58 AM, Andrew Cooper wrote:
On 17/02/17 14:17, Boris Ostrovsky wrote:

On 02/17/2017 03:27 AM, Jan Beulich wrote:
On 16.02.17 at 19:09, <boris.ostrovsky@xxxxxxxxxx> wrote:
On 02/16/2017 12:09 PM, Andrew Cooper wrote:
Second, if it is available, has the toolstack chosen to allow the
to use it.  This should determine whether features/information are
visible in CPUID.

You mean if toolstack masks out leaf 0xa on Intel? I chould check this
in get_vpmu(). Is this information available by the time
vcpu_initialise() runs?

You shouldn't rely on this, even if it happened to be that way
right now. Instead you'd have to adjust accordingly when CPUID
info gets updated by the tool stack (update_domain_cpuid_info()
as the root hook point).

Right, that's what I was going to do --- destroy VPMU if CPUID
indicates no support.

From a CPUID side of things, the way I would eventually like things to
work is this:

* Hardware support and Xen command line options influence the visibility
of vPMU bits in {hvm,pv}_max_policy.

* Using *_max_policy and domain.cfg settings, a toolstack opts in to
allowing vPMU by setting vPMU details in the domains policy.

The eventual plan for toolstack API will be that the entire policy is
got/set at once (rather than individual leaves as it currently exists).
Xen audits the toolstacks choice against {hvm,pv}_max_policy, rejecting
the update wholesale if it is bad.

Then, Xen will compare the old and proposed new policies to see what has
changed, and use this to enable subsystems for a domain (either
passively by having the subsystem rely on CPUID values, or actively by
calling into the subsystem to initialise things).  nested-virt was my
main target here, but vPMU looks like it fits into the same category, as
well as some of the debugging facilities such as LBR etc.

I realise that this infrastructure doesn't all exist yet,  but it would
be helpful if your fix can easily be turned into API matching the above,
when I complete the missing CPUID pieces.

Currently, since we don't know whether the toolstack will be updating leaf 0xa, we have to initialize VPMU at VCPU initialization time and then possibly destroy it if CPUID update does come in.

When your suggested infrastructure is in place it we can defer VPMU initialization until after the policy has been loaded. And I think it should be a pretty easy conversion.

Let me send what I have now.


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