Re: [Xen-devel] [PATCH v8 3/9] xen/x86: support per-domain flag for xpti

[Sergey Dyasli <sergey.dyasli@xxxxxxxxxx>]

Hi Juergen,

2 small requests from me below.

On Wed, 2018-04-18 at 10:30 +0200, Juergen Gross wrote:
> Instead of switching XPTI globally on or off add a per-domain flag for
> that purpose. This allows to modify the xpti boot parameter to support
> running dom0 without Meltdown mitigations. Using "xpti=nodom0" as boot
> parameter will achieve that.
> 
> Move the xpti boot parameter handling to xen/arch/x86/pv/domain.c as
> it is pv-domain specific.
> 
> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>


> diff --git a/docs/misc/xen-command-line.markdown 
> b/docs/misc/xen-command-line.markdown
> index b353352adf..d4f758487a 100644
> --- a/docs/misc/xen-command-line.markdown
> +++ b/docs/misc/xen-command-line.markdown
> @@ -1955,14 +1955,24 @@ 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)

Could this line please be changed to:

    `false` on hardware known not to be 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.
> +
> +With `dom0` and `domu` it is possible to control page table isolation
> +for dom0 or guest domains only.
> +
>  ### xsave
>  > `= <boolean>`
>  

> diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
> index 5ab4ff3f68..b4fa43269e 100644
> --- 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 OPT_XPTI_DOM0  0x01
> +#define OPT_XPTI_DOMU  0x02
> +
>  static inline void init_shadow_spec_ctrl_state(void)
>  {
>      struct cpu_info *info = get_cpu_info();

Could you please also include something like the following:

@@ -119,8 +122,9 @@ static void __init print_details(enum ind_thunk thunk)
            boot_cpu_has(X86_FEATURE_RSB_NATIVE)      ? " RSB_NATIVE" : "",
            boot_cpu_has(X86_FEATURE_RSB_VMEXIT)      ? " RSB_VMEXIT" : "");
 
-    printk("XPTI: %s\n",
-           boot_cpu_has(X86_FEATURE_NO_XPTI) ? "disabled" : "enabled");
+    printk("XPTI: Dom0 %s, DomU (64-bit PV only) %s\n",
+           opt_xpti & OPT_XPTI_DOM0 ? "enabled" : "disabled",
+           opt_xpti & OPT_XPTI_DOMU ? "enabled" : "disabled");
 }


(just noticed that commit message also needs update regarding param name)

-- 
Thanks,
Sergey
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