-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 On 01/27/2015 03:27 PM, Rik van Riel wrote: > On 01/27/2015 02:40 PM, Oleg Nesterov wrote:
>>>> - Why unlazy_fpu() always does __save_init_fpu() even if >>>> use_eager_fpu? >>>> >>>> and note that in this case __thread_fpu_end() is wrong if >>>> use_eager_fpu, but fortunately the only possible caller of >>>> unlazy_fpu() is coredump. fpu_copy() checks use_eager_fpu(). >>>> >>>> - Is unlazy_fpu()->__save_init_fpu() safe wrt >>>> __kernel_fpu_begin() from irq? > > It looks like it should be safe, as long as __save_init_fpu() knows > that the task no longer has the FPU after __kernel_fpu_end(), so it > does not try to save the kernel FPU state to the user's > task->thread.fpu.state->xstate > > The caveat here is that __kernel_fpu_begin()/__kernel_fpu_end() > needs to be kept from running during unlazy_fpu(). > > This means interrupted_kernel_fpu_idle and/or irq_fpu_usable need > to check whether preemption is disabled, and lock out > __kernel_fpu_begin() when preemption is disabled. > > It does not look like it currently does that... ... and that won't work, because preempt_disable() is a noop without CONFIG_PREEMPT enabled. Sigh. Not sure how to work around that, except by having __Kernel_fpu_end() always restore the task FPU state, if the task had the FPU when entering. This suggests it may be advantageous to be liberal with __thread_fpu_end() when we know user space will not be touching the FPU again for a while, since that would reduce the work done by __kernel_fpu_begin()/__kernel_fpu_end(). Does that make sense? - -- All rights reversed -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJUx/oJAAoJEM553pKExN6Dh4YH/AtVOza5VeZuz55s59LifpLt 5eXMQBxQt0JiX9NACc+7qXDuEFfkmnzhNpNT98abuic7/1dvj2TfcoXSnkysDaNb WojFW0gMc973G4UA6ZPsqScxz7jdfaqNrFdFEaTPM6URClIllVXTgnCcDoWCFfR2 627s6vyOW//BgXnedHraGGqnBVpCBLWrsVnhJ0RbPd2cjrrwKPXcbLnk6+uzZqLY /7Zp2fSDjuAY6PhxRZM529vyJNqIM2mhlKFG8/xnch95mlXwtOQvLuuJmkwbKabb 6Z1gqpyEXbWyB2riD/pQrIlkOg2P4UuLONM7fgFY4Dzc8MEM7UmNR4mWoPAeBuU= =SSqD -----END PGP SIGNATURE----- -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
