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

2017-05-11 Thread Thomas Gleixner 
On Thu, 11 May 2017, Juergen Gross wrote:

> 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 
> 
> Any objections for carrying this through the Xen tree?


Acked-by: Thomas Gleixner 

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

2017-05-11 Thread Juergen Gross 
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 

Any objections for carrying this through the Xen tree?


Juergen

> ---
>  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);
> 


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

2017-05-05 Thread Boris Ostrovsky 
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 



>>
>> Signed-off-by: Juergen Gross 
>> ---
>>  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);
>>


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

2017-05-04 Thread Juergen Gross 
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 
> ---
>  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);
> 


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

2017-04-26 Thread Juergen Gross 
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 
---
 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);
-- 
2.12.0


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