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    SubjectRe: [patch V3 34/66] x86/fpu: Get rid of the FNSAVE optimization
    On Fri, Jun 18, 2021 at 04:18:57PM +0200, Thomas Gleixner wrote:
    > The FNSAVE support requires conditionals in quite some call paths because
    > FNSAVE reinitialized the FPU hardware. If the save has to preserve the FPU

    reinitializes

    > 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>
    > ---
    > V3: New patch
    > ---
    > arch/x86/include/asm/fpu/internal.h | 17 +++++++----
    > arch/x86/kernel/fpu/core.c | 54 +++++++++++++++---------------------
    > 2 files changed, 34 insertions(+), 37 deletions(-)
    >
    > --- a/arch/x86/include/asm/fpu/internal.h
    > +++ b/arch/x86/include/asm/fpu/internal.h
    > @@ -375,7 +375,7 @@ static inline int os_xrstor_safe(struct
    > return err;
    > }
    >
    > -extern int save_fpregs_to_fpstate(struct fpu *fpu);
    > +extern void save_fpregs_to_fpstate(struct fpu *fpu);

    You can move that stray forward declaration up, to the others.

    ...

    > --- a/arch/x86/kernel/fpu/core.c
    > +++ b/arch/x86/kernel/fpu/core.c
    > @@ -83,16 +83,20 @@ bool irq_fpu_usable(void)
    > EXPORT_SYMBOL(irq_fpu_usable);
    >
    > /*
    > - * These must be called with preempt disabled. Returns
    > - * 'true' if the FPU state is still intact and we can
    > - * keep registers active.
    > + * Save the FPU register state in fpu->state. The register state is
    > + * preserved.
    > *
    > - * The legacy FNSAVE instruction cleared all FPU state
    > - * unconditionally, so registers are essentially destroyed.
    > - * Modern FPU state can be kept in registers, if there are
    > - * no pending FP exceptions.
    > + * Must be called with fpregs_lock() held.
    > + *
    > + * The legacy FNSAVE instruction clears all FPU state unconditionally, so
    > + * register state has to be reloaded. That might be a pointless exercise
    > + * when the FPU is going to be used by another task right after that. But
    > + * this only affect 20+ years old 32bit systems and avoids conditionals all

    affects

    > + * over the place.
    > + *
    > + * FXSAVE and all XSAVE variants preserve the FPU register state.
    > */
    > -int save_fpregs_to_fpstate(struct fpu *fpu)
    > +void save_fpregs_to_fpstate(struct fpu *fpu)

    With that addressed:

    Reviewed-by: Borislav Petkov <bp@suse.de>

    --
    Regards/Gruss,
    Boris.

    SUSE Software Solutions Germany GmbH, GF: Felix Imendörffer, HRB 36809, AG Nürnberg

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