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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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). 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?

>> 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?

Jan


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