[PATCH] x86/speculation: Revert turning on STIBP all the time

[PATCH] x86/speculation: Revert turning on STIBP all the time

[Tim Chen]

Commit 53c613fe "x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation"
turns on STIBP all the time.

This causes large performance regression in many workloads.

One case is perlbench in the SpecInt Rate 2006 test suite which shows a
21% reduction in throughput.
There're also other reports of drop in performance on Python and PHP benchmarks:
https://www.phoronix.com/scan.php?page=article&item=linux-420-bisect&num=2

STIBP on all the time should not be the default option.

Turn off STIBP all the time for now till STIBP can be applied on
a per task basis.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
---
 arch/x86/kernel/cpu/bugs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index c37e66e..21a8f39 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -332,7 +332,7 @@ static bool stibp_needed(void)
 	if (!boot_cpu_has(X86_FEATURE_STIBP))
 		return false;
 
-	return true;
+	return false;
 }
 
 static void update_stibp_msr(void *info)
-- 
2.9.4

Re: [PATCH] x86/speculation: Revert turning on STIBP all the time

[Jiri Kosina]

On Wed, 21 Nov 2018, Tim Chen wrote:

> Commit 53c613fe "x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation"
> turns on STIBP all the time.
> 
> This causes large performance regression in many workloads.
> 
> One case is perlbench in the SpecInt Rate 2006 test suite which shows a
> 21% reduction in throughput.
> There're also other reports of drop in performance on Python and PHP benchmarks:
> https://www.phoronix.com/scan.php?page=article&item=linux-420-bisect&num=2
> 
> STIBP on all the time should not be the default option.
> 
> Turn off STIBP all the time for now till STIBP can be applied on
> a per task basis.
> 
> Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> ---
>  arch/x86/kernel/cpu/bugs.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index c37e66e..21a8f39 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -332,7 +332,7 @@ static bool stibp_needed(void)
>  	if (!boot_cpu_has(X86_FEATURE_STIBP))
>  		return false;
>  
> -	return true;
> +	return false;
>  }

For -stable, which actually makes it to production, I already asked Greg 
to drop it.

For -rc, I don't think we need to do this at this moment, given the 
prctl+seccomp fixup is basically ready, do we?

Thanks,

-- 
Jiri Kosina
SUSE Labs

Re: [PATCH] x86/speculation: Revert turning on STIBP all the time

[Linus Torvalds]

On Wed, Nov 21, 2018 at 12:51 PM Jiri Kosina <jikos@kernel.org> wrote:
>
> For -rc, I don't think we need to do this at this moment, given the
> prctl+seccomp fixup is basically ready, do we?

Agreed.

               Linus

Re: [PATCH] x86/speculation: Revert turning on STIBP all the time

[Greg KH]

On Wed, Nov 21, 2018 at 09:51:19PM +0100, Jiri Kosina wrote:
> On Wed, 21 Nov 2018, Tim Chen wrote:
> 
> > Commit 53c613fe "x86/speculation: Enable cross-hyperthread spectre v2 STIBP mitigation"
> > turns on STIBP all the time.
> > 
> > This causes large performance regression in many workloads.
> > 
> > One case is perlbench in the SpecInt Rate 2006 test suite which shows a
> > 21% reduction in throughput.
> > There're also other reports of drop in performance on Python and PHP benchmarks:
> > https://www.phoronix.com/scan.php?page=article&item=linux-420-bisect&num=2
> > 
> > STIBP on all the time should not be the default option.
> > 
> > Turn off STIBP all the time for now till STIBP can be applied on
> > a per task basis.
> > 
> > Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
> > ---
> >  arch/x86/kernel/cpu/bugs.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index c37e66e..21a8f39 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -332,7 +332,7 @@ static bool stibp_needed(void)
> >  	if (!boot_cpu_has(X86_FEATURE_STIBP))
> >  		return false;
> >  
> > -	return true;
> > +	return false;
> >  }
> 
> For -stable, which actually makes it to production, I already asked Greg 
> to drop it.

It's in the latest stable -rc releases, and if all goes well, they will
be released tomorrow.

thanks,

greg k-h