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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 02/16/2017 12:09 PM, Andrew Cooper wrote:
> On 16/02/17 16:59, Jan Beulich wrote:
> > > > > On 16.02.17 at 15:59, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > > vpmu_enabled() (used by hvm/pv_cpuid() to properly report 0xa leaf
> > > for Intel processors) is based on the value of VPMU_CONTEXT_ALLOCATED
> > > bit. This is problematic:
> > > * For HVM guests VPMU context is allocated lazily, during the first
> > >   access to VPMU MSRs. Since the leaf is typically queried before guest
> > >   attempts to read or write the MSRs it is likely that CPUID will report
> > >   no PMU support
> > > * For PV guests the context is allocated eagerly but only in responce to
> > >   guest's XENPMU_init hypercall. There is a chance that the guest will
> > >   try to read CPUID before making this hypercall.
> > >
> > > This patch introduces VPMU_ENABLED flag which is set (subject to vpmu_mode
> > > constraints) during VCPU initialization for both PV and HVM guests. Since
> > > this flag is expected to be managed together with vpmu_count, get/put_vpmu()
> > > are added to simplify code.
> > I think VPMU_ENABLED is misleading, as it may as well mean the state
> > after the guest did enable it. How about VPMU_AVAILABLE?
> The problem is a little deeper than that.
>
> First, there is whether it is available based on hypervisor configuration.
This bit is set only if vpmu_mode permits it.

> Second, if it is available, has the toolstack chosen to allow the domain
> 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?
> Finally, if vpmu is permitted, has the domain turned it on.
HVM domains always do and PV domains essentially too.


-boris

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