linux-kernel - Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS

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Date:   Fri, 5 May 2017 12:02:02 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Juergen Gross <jgross@...e.com>, linux-kernel@...r.kernel.org,
        xen-devel@...ts.xenproject.org, x86@...nel.org
Cc:     hpa@...or.com, tglx@...utronix.de, mingo@...hat.com
Subject: Re: [PATCH v3] x86/amd: don't set X86_BUG_SYSRET_SS_ATTRS when
 running under Xen

On 05/05/2017 01:09 AM, Juergen Gross wrote:
> 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.

FWIW,

Reviewed-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>



>>
>> Signed-off-by: Juergen Gross <jgross@...e.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);
>>