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

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.


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