Re: [Xen-devel] [PATCH v6 3/9] xen/x86: support per-domain flag for xpti

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.04.18 at 09:58, <jgross@xxxxxxxx> wrote:
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -1955,14 +1955,29 @@ clustered mode.  The default, given no hint from the 
> **FADT**, is cluster
>  mode.
>  
>  ### xpti
> -> `= <boolean>`
> +> `= List of [ default | <boolean> | dom0=<bool> | domu=<bool> ]`
>  
> -> Default: `false` on AMD hardware
> +> Default: `false` on hardware not vulnerable to Meltdown (e.g. AMD)
>  > Default: `true` everywhere else
>  
>  Override default selection of whether to isolate 64-bit PV guest page
>  tables.
>  
> +`true` activates page table isolation even on hardware not vulnerable by
> +Meltdown for all domains.
> +
> +`false` deactivates page table isolation on all systems for all domains.
> +
> +`default` sets the default behaviour.
> +
> +`dom0=false` deactivates page table isolation for dom0.
> +
> +`dom0=true` activates page table isolation for dom0.
> +
> +`domu=false` deactivates page table isolation for guest domains.
> +
> +`domu=true` activates page table isolation for guest domains.

This is too verbose / repetitive for my taste.

> @@ -205,6 +208,10 @@ int pv_domain_initialise(struct domain *d)
>      /* 64-bit PV guest by default. */
>      d->arch.is_32bit_pv = d->arch.has_32bit_shinfo = 0;
>  
> +    d->arch.pv_domain.xpti = (d->domain_id == hardware_domid)
> +                             ? (opt_xpti & XPTI_DOM0)
> +                             : (opt_xpti & XPTI_DOMU);

I would generally prefer to have as little redundancy as possible in
such expressions, i.e.

    d->arch.pv_domain.xpti = opt_xpti & (d->domain_id == hardware_domid
                                         ? XPTI_DOM0 : XPTI_DOMU);

Furthermore - shouldn't this cover domain 0 as well as the hardware
domain, even if - in case they are different - domain 0 should be
short lived?

> --- a/xen/arch/x86/spec_ctrl.c
> +++ b/xen/arch/x86/spec_ctrl.c
> @@ -193,6 +193,68 @@ static bool __init retpoline_safe(void)
>     }
> }
> 
> +#define XPTI_DEFAULT  0xff
> +uint8_t opt_xpti = XPTI_DEFAULT;

__read_mostly

> --- a/xen/include/asm-x86/spec_ctrl.h
> +++ b/xen/include/asm-x86/spec_ctrl.h
> @@ -29,6 +29,10 @@ void init_speculation_mitigations(void);
>  extern bool opt_ibpb;
>  extern uint8_t default_bti_ist_info;
>  
> +extern uint8_t opt_xpti;
> +#define XPTI_DOM0  0x01
> +#define XPTI_DOMU  0x02

OPT_XPTI_DOM{0,U} would perhaps have been better.

Anyway, in the interest of getting done with this
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with or without some or all of the suggestions addressed.

Jan


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