Re: [Xen-devel] [PATCH for-4.6] p2m/ept: Set the A bit only if PML is enabled

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 24.09.15 at 11:13, <andrew.cooper3@xxxxxxxxxx> wrote:

>>> etc along with adjusting the existing gating of PML on AD being
>>> available (perhaps by simply stripping the respective bit from what
>>> we read from MSR_IA32_VMX_EPT_VPID_CAP). Of course this
>>> then ignores the fact that the erratum only affects the A bit, but
>>> I think we can live with that.
>>>
>>> I also think the currently slightly strange setting of the ept_ad bit
>>> should be changed: There's no point setting the bit for domains
>>> not getting PML enabled (and incurring the overhead of the
>>> hardware updating the bits); imo this should instead be done in
>>> ept_enable_pml() / vmx_domain_enable_pml() (and undone in
>>> the respective disable function).
>> Yep.
> 
> Just as a note, in the non PML case, the AD enable bit in EPTP is left 
> clear, which means that the A/D bits in the EPTs have no effect.

Not exactly: eptp.ad gets turned on when the hardware supports
PML, not when the guest gets PML enabled. I.e. for all the time the
guest runs without PML enabled there still is A/D checking overhead
(yet, because of the way we set them by default, there may not
be any page table updates by the hardware). Of course this may,
depending on how it's actually implemented in hardware, not mean
any performance effect at all.

Jan


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