Messages in this thread | | | From | Andy Lutomirski <> | Date | Wed, 17 Jun 2020 08:30:42 -0700 | Subject | Re: [PATCH] x86/fpu: Reset MXCSR to default in kernel_fpu_begin() |
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On Wed, Jun 17, 2020 at 1:33 AM Borislav Petkov <bp@alien8.de> wrote: > > On Tue, Jun 16, 2020 at 02:17:16PM -0700, Andy Lutomirski wrote: > > We definitely need to sanitize MXCSR for kernel fpu if kernel fpu > > means SSE2. If kernel fpu means x87, we need to fix the fpu control > > word. > > Bah, there's no need to beat around the bush - let's just do: > > if (boot_cpu_has(X86_FEATURE_XMM)) > ldmxcsr(MXCSR_DEFAULT); > > if (boot_cpu_has(X86_FEATURE_FPU)) > asm volatile ("fninit"); > > and be sure that kernel users get a squeaky-clean FPU. > > > On x86_64, I suspect the UEFI ABI technically requires a clean x87 > > control word too. If we’re willing to declare that the kernel proper > > won’t use x87, then we could shove that into the UEFI code. > > Nah, we don't trust the firmware.
What I mean is: if we trust ourselves to have no x87 instructions in the kernel, we could put the FNINIT in the UEFI stubs to save some cycles. I don't know how slow FNINIT is.
> > -- > Regards/Gruss, > Boris. > > https://people.kernel.org/tglx/notes-about-netiquette
-- Andy Lutomirski AMA Capital Management, LLC
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