x86/fpu: Get rid of the FNSAVE optimization

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The FNSAVE support requires conditionals in quite some call paths because
FNSAVE reinitializes the FPU hardware. If the save has to preserve the FPU
register state then the caller has to conditionally restore it from memory
when FNSAVE is in use.

This also requires a conditional in context switch because the restore
avoidance optimization cannot work with FNSAVE. As this only affects 20+
years old CPUs there is really no reason to keep this optimization
effective for FNSAVE. It's about time to not optimize for antiques anymore.

Just unconditionally FRSTOR the save content to the registers and clean up
the conditionals all over the place.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121454.617369268@linutronix.de

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Thomas Gleixner 2021-06-23 14:02:00 +02:00 • committed by Borislav Petkov

parent ebe7234b08

commit 08ded2cd18

2 changed files with 34 additions and 38 deletions

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18

arch/x86/include/asm/fpu/internal.h

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54

arch/x86/kernel/fpu/core.c

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