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

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

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).

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

-boris

>
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index 045f6ff..834ce0f 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -721,10 +721,12 @@ static int __init init_pv_cr4_masks(void)
>       unsigned long common_mask = ~X86_CR4_TSD;
>   
>       /*
> -     * All PV guests may attempt to modify TSD, DE and OSXSAVE.
> +     * All PV guests may attempt to modify TSD, DE, PCE and OSXSAVE.
>        */
>       if ( cpu_has_de )
>           common_mask &= ~X86_CR4_DE;
> +    if ( cpu_has_pce )
> +        common_mask &= ~X86_CR4_PCE;
>       if ( cpu_has_xsave )
>           common_mask &= ~X86_CR4_OSXSAVE;
>   
> ~Andrew