Re: [Xen-devel] [PATCH 3/9] x86/mm: honor opt_pcid also for 32-bit PV domains

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Sep 11, 2019 at 05:22:51PM +0200, Jan Beulich wrote:
> I can't see any technical or performance reason why we should treat
> 32-bit PV different from 64-bit PV in this regard.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

The original commit mentions that PCID doesn't improve performance for
non-XPTI domains, but it doesn't mention whether it makes performance
worse. The change LGTM, if you are fine performance wise:

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> 
> --- a/xen/arch/x86/pv/domain.c
> +++ b/xen/arch/x86/pv/domain.c
> @@ -180,7 +180,24 @@ int switch_compat(struct domain *d)
>      d->arch.x87_fip_width = 4;
>  
>      d->arch.pv.xpti = false;
> -    d->arch.pv.pcid = false;
> +
> +    if ( use_invpcid && cpu_has_pcid )
> +        switch ( ACCESS_ONCE(opt_pcid) )
> +        {
> +        case PCID_OFF:
> +        case PCID_XPTI:
> +            d->arch.pv.pcid = false;
> +            break;
> +
> +        case PCID_ALL:
> +        case PCID_NOXPTI:
> +            d->arch.pv.pcid = true;
> +            break;
> +
> +        default:
> +            ASSERT_UNREACHABLE();
> +            break;
> +        }

This chunk is (functionality wise) exactly the same as the one in
pv_domain_initialise, it might be good to put this in a separate
helper?

>  
>      return 0;
>  
> @@ -312,7 +329,7 @@ int pv_domain_initialise(struct domain *
>  
>      d->arch.pv.xpti = is_hardware_domain(d) ? opt_xpti_hwdom : opt_xpti_domu;
>  
> -    if ( !is_pv_32bit_domain(d) && use_invpcid && cpu_has_pcid )

This is_pv_32bit_domain is already pointless, is_32bit_pv gets
unconditionally set to 0 just two lines above.

Thanks, Roger.

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