Re: [PATCH 10/10] x86/spec_ctrl: Elide MSR_SPEC_CTRL handling in idle context when possible

[Wei Liu <wei.liu2@citrix.com>]

On Fri, May 11, 2018 at 11:38:14AM +0100, Andrew Cooper wrote:
> If Xen is virtualising MSR_SPEC_CTRL handling for guests, but using 0 as its
> own MSR_SPEC_CTRL value, spec_ctrl_{enter,exit}_idle() need not write to the
> MSR.
> 
> Requested-by: Jan Beulich <JBeulich@suse.com>
> Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
> ---
> CC: Jan Beulich <JBeulich@suse.com>
> CC: Wei Liu <wei.liu2@citrix.com>
> CC: Roger Pau Monné <roger.pau@citrix.com>
> CC: Juergen Gross <jgross@suse.com>
> ---
>  xen/arch/x86/spec_ctrl.c          | 4 ++++
>  xen/include/asm-x86/cpufeatures.h | 1 +
>  xen/include/asm-x86/spec_ctrl.h   | 8 ++------
>  3 files changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
> index a2328bd..f4a3165 100644
> --- a/xen/arch/x86/spec_ctrl.c
> +++ b/xen/arch/x86/spec_ctrl.c
> @@ -526,6 +526,10 @@ void __init init_speculation_mitigations(void)
>      /* (Re)init BSP state now that default_spec_ctrl_flags has been calculated. */
>      init_shadow_spec_ctrl_state();
>  
> +    /* If Xen is using any MSR_SPEC_CTRL settings, adjust the idle path. */
> +    if ( default_xen_spec_ctrl )
> +        setup_force_cpu_cap(X86_FEATURE_SC_MSR_IDLE);
> +
>      xpti_init_default(false);
>      if ( opt_xpti == 0 )
>          setup_force_cpu_cap(X86_FEATURE_NO_XPTI);
> diff --git a/xen/include/asm-x86/cpufeatures.h b/xen/include/asm-x86/cpufeatures.h
> index 9d5d81e..b90aa2d 100644
> --- a/xen/include/asm-x86/cpufeatures.h
> +++ b/xen/include/asm-x86/cpufeatures.h
> @@ -31,3 +31,4 @@ XEN_CPUFEATURE(SC_MSR_HVM,      (FSCAPINTS+0)*32+17) /* MSR_SPEC_CTRL used by Xe
>  XEN_CPUFEATURE(SC_RSB_PV,       (FSCAPINTS+0)*32+18) /* RSB overwrite needed for PV */
>  XEN_CPUFEATURE(SC_RSB_HVM,      (FSCAPINTS+0)*32+19) /* RSB overwrite needed for HVM */
>  XEN_CPUFEATURE(NO_XPTI,         (FSCAPINTS+0)*32+20) /* XPTI mitigation not in use */
> +XEN_CPUFEATURE(SC_MSR_IDLE,     (FSCAPINTS+0)*32+21) /* (SC_MSR_PV || SC_MSR_HVM) && default_xen_spec_ctrl */

I don't follow: the code above only depends on default_xen_spec_ctrl but
the comment says SC_MSR_PV and SC_MSR_HVM are also taken into
consideration.

The rest looks fine.

Wei.

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