Re: [Xen-devel] [PATCH v3 1/2] x86/vpmu: Add get/put_vpmu() and VPMU_AVAILABLE

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.02.17 at 19:24, <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_AVAILABLE 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.
> 
> vpmu_enabled() (renamed to vpmu_available()) can now use this new flag.
> 
> (As a side affect this patch also fixes a race in pvpmu_init() where we
> increment vcpu_count in vpmu_initialise() after checking vpmu_mode)
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>



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