lkml.org 
[lkml]   [2016]   [Jan]   [23]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [PATCH 1/5] x86/fpu: Fix math emulation in eager fpu mode
On Sat, Jan 23, 2016 at 2:02 AM, Borislav Petkov <bp@alien8.de> wrote:
> On Fri, Jan 22, 2016 at 04:56:02PM -0800, Andy Lutomirski wrote:
>> Systems without an FPU are generally old and therefore use lazy FPU
>> switching. Unsurprisingly, math emulation in eager FPU mode is a
>> bit buggy. Fix it.
>>
>> There were two bugs involving kernel code trying to use the FPU
>> registers in eager mode even if they didn't exist and one BUG_ON
>> that was incorrect.
>>
>> Signed-off-by: Andy Lutomirski <luto@kernel.org>
>> ---
>> arch/x86/include/asm/fpu/internal.h | 3 ++-
>> arch/x86/kernel/fpu/core.c | 2 +-
>> arch/x86/kernel/traps.c | 1 -
>> 3 files changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h
>> index 0fd440df63f1..a1f78a9fbf41 100644
>> --- a/arch/x86/include/asm/fpu/internal.h
>> +++ b/arch/x86/include/asm/fpu/internal.h
>> @@ -589,7 +589,8 @@ switch_fpu_prepare(struct fpu *old_fpu, struct fpu *new_fpu, int cpu)
>> * If the task has used the math, pre-load the FPU on xsave processors
>> * or if the past 5 consecutive context-switches used math.
>> */
>> - fpu.preload = new_fpu->fpstate_active &&
>> + fpu.preload = static_cpu_has(X86_FEATURE_FPU) &&
>> + new_fpu->fpstate_active &&
>> (use_eager_fpu() || new_fpu->counter > 5);
>
> Should we move that static_cpu_has(X86_FEATURE_FPU) check in
> use_eager_fpu()?

I'm confused. I want to make use_eager_fpu() return true always, so
how would this help? The idea is for FPU emulation to work despite
being in eager mode.

>
> I mean, when !X86_FEATURE_FPU, then we most certainly aren't doing eager
> FPU anyway.

With this patch applied, we can do eager fpu without X86_FEATURE_FPU :)

--Andy

\
 
 \ /
  Last update: 2016-01-23 19:21    [W:0.236 / U:0.120 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site