Re: [PATCH] x86: Allow PV guest set X86_CR4_PCE flag

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 10/08/15 15:49, Boris Ostrovsky wrote:
>
>
> On 08/10/2015 10:37 AM, Andrew Cooper wrote:
>> On 10/08/15 15:27, Boris Ostrovsky wrote:
>>> With added PV support for VPMU, guests may legitimately decide to set
>>> CR4's PCE flag. We should allow this when VPMU is enabled.
>>>
>>> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>> Why?  Even a PV guest using VPMU should know that it doesn't actually
>> control CR4.PCE
>>
>> All this (appears to) end up doing is putting PCE into the "allow but
>> ignore" mask.
>
> Yes, that's what I wanted to do.
>
>>
>> How about this (not even compile tested) which is a rather shorter way
>> of doing the same thing:
>
> We could do this too but I thought that if we have VPMU off there is
> no reason to allow this bit to be set (quietly).

Adding the bit to this mask doesn't allow the guest to play with it. 
Xen never sets CR4.PCE.

The question is whether warning about the guest attempting to set it is
worthwhile or not.  We have no rdpmc support in emulate_privileged_op()
so any attempt to use it will result in a #UD being injected.

>
> (There is no cpu_has_pce, we'd use cpu_has_arch_perfmon on Intel and
> do this unconditionally on AMD)

It could trivially be added.  The salient piece of information is
whether the hardware would support setting CR4.PCE, not whether any of
the interesting features which come with it are present.

~Andrew