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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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 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?
> 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.

> Which gets us to another point: Shouldn't
> we disallow CPUID info updates after the guest started running?
> Or do we mean to trust the tool stack to not do this if it doesn't
> want to confuse a guest?
I believe currently it's the latter. I don't see anything preventing 
CPUID update to a running guest (except for pausing it while the update 
is in progress).
> > > Finally, if vpmu is permitted, has the domain turned it on.
> > HVM domains always do and PV domains essentially too.
> How that? Or are you talking of just modern Linux guests?
The newer ones.


-boris

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