Re: [PATCH 1/5] x86/fpu: Fix math emulation in eager fpu mode

[Andy Lutomirski]

On Sat, Jan 23, 2016 at 2:02 AM, Borislav Petkov <bp@xxxxxxxxx> 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@xxxxxxxxxx>
>> ---
>>  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