Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen

From	Juergen Gross <jgross@suse.com>
Newsgroups	linux.kernel
Subject	Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen
Date	2017-05-05 07:10 +0200
Message-ID	<tDDF7-5EL-1@gated-at.bofh.it> (permalink)
References	<tAJQK-4gp-7@gated-at.bofh.it>
Organization	linux.* mail to news gateway

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Any comments?


Juergen

On 27/04/17 07:01, Juergen Gross wrote:
> When running as Xen pv guest X86_BUG_SYSRET_SS_ATTRS must not be set
> on AMD cpus.
> 
> This bug/feature bit is kind of special as it will be used very early
> when switching threads. Setting the bit and clearing it a little bit
> later leaves a critical window where things can go wrong. This time
> window has enlarged a little bit by using setup_clear_cpu_cap() instead
> of the hypervisor's set_cpu_features callback. It seems this larger
> window now makes it rather easy to hit the problem.
> 
> The proper solution is to never set the bit in case of Xen.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
>  arch/x86/kernel/cpu/amd.c   | 5 +++--
>  arch/x86/xen/enlighten_pv.c | 1 -
>  2 files changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
> index c36140d788fe..b6da6e75e3a8 100644
> --- a/arch/x86/kernel/cpu/amd.c
> +++ b/arch/x86/kernel/cpu/amd.c
> @@ -799,8 +799,9 @@ static void init_amd(struct cpuinfo_x86 *c)
>  		if (cpu_has(c, X86_FEATURE_3DNOW) || cpu_has(c, X86_FEATURE_LM))
>  			set_cpu_cap(c, X86_FEATURE_3DNOWPREFETCH);
>  
> -	/* AMD CPUs don't reset SS attributes on SYSRET */
> -	set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
> +	/* AMD CPUs don't reset SS attributes on SYSRET, Xen does. */
> +	if (!cpu_has(c, X86_FEATURE_XENPV))
> +		set_cpu_bug(c, X86_BUG_SYSRET_SS_ATTRS);
>  }
>  
>  #ifdef CONFIG_X86_32
> diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> index a1af4f68278f..dcfd07faf1c3 100644
> --- a/arch/x86/xen/enlighten_pv.c
> +++ b/arch/x86/xen/enlighten_pv.c
> @@ -290,7 +290,6 @@ static bool __init xen_check_xsave(void)
>  
>  static void __init xen_init_capabilities(void)
>  {
> -	setup_clear_cpu_cap(X86_BUG_SYSRET_SS_ATTRS);
>  	setup_force_cpu_cap(X86_FEATURE_XENPV);
>  	setup_clear_cpu_cap(X86_FEATURE_DCA);
>  	setup_clear_cpu_cap(X86_FEATURE_APERFMPERF);
>

Thread

[PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when running under Xen Juergen Gross <jgross@suse.com> - 2017-04-27 07:10 +0200
  Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when  running under Xen Juergen Gross <jgross@suse.com> - 2017-05-05 07:10 +0200
    Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when  running under Xen Boris Ostrovsky <boris.ostrovsky@oracle.com> - 2017-05-05 18:10 +0200
  Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when  running under Xen Juergen Gross <jgross@suse.com> - 2017-05-11 12:40 +0200
    Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when  running under Xen Thomas Gleixner <tglx@linutronix.de> - 2017-05-11 14:50 +0200

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