Messages in this thread | | | From | Andy Lutomirski <> | Date | Sat, 23 Jan 2016 09:40:06 -0800 | Subject | Re: [PATCH 1/5] x86/fpu: Fix math emulation in eager fpu mode |
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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
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