Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore_sig()

Yu-cheng Yu
2020-03-02T18:10:26Z

On Sat, 2020-02-29 at 15:36 +0100, Borislav Petkov wrote:
> On Fri, Feb 28, 2020 at 02:13:29PM -0800, Yu-cheng Yu wrote:
> > If the XSAVES buffer already has current data (i.e. TIF_NEED_FPU_LOAD is
> > set), then skip copy_xregs_to_kernel(). This happens when the task was
> > context-switched out and has not returned to user-mode.
>
> So I got tired of this peacemeal game back'n'forth and went and did your
> work for ya.
>
> First of all, on my fairly new KBL test box, the context size is almost
> a kB:
>
> [ 0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
> [ 0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
> [ 0.000000] x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
> [ 0.000000] x86/fpu: Supporting XSAVE feature 0x008: 'MPX bounds registers'
> [ 0.000000] x86/fpu: Supporting XSAVE feature 0x010: 'MPX CSR'
> [ 0.000000] x86/fpu: xstate_offset[2]: 576, xstate_sizes[2]: 256
> [ 0.000000] x86/fpu: xstate_offset[3]: 832, xstate_sizes[3]: 64
> [ 0.000000] x86/fpu: xstate_offset[4]: 896, xstate_sizes[4]: 64
> [ 0.000000] x86/fpu: Enabled xstate features 0x1f, context size is 960 bytes, using 'compacted' format.
>
> Then, I added this ontop of your patchset:
>
> diff --git a/arch/x86/kernel/fpu/signal.c b/arch/x86/kernel/fpu/signal.c
> index 0d3e06a772b0..2e57b8d79c0e 100644
> --- a/arch/x86/kernel/fpu/signal.c
> +++ b/arch/x86/kernel/fpu/signal.c
> @@ -337,6 +337,8 @@ static int __fpu__restore_sig(void __user *buf, void __user *buf_fx, int size)
> */
> fpregs_lock();
> if (!test_thread_flag(TIF_NEED_FPU_LOAD)) {
> + trace_printk("!NEED_FPU_LOAD, size: %d, supervisor: 0x%llx\n",
> + size, xfeatures_mask_supervisor());
> if (xfeatures_mask_supervisor())
> copy_xregs_to_kernel(&fpu->state.xsave);
> set_thread_flag(TIF_NEED_FPU_LOAD);
>
> and traced a fairly boring kernel build workload where the kernel
> .config is not even a distro one but a tailored for this machine.
>
> Which means, it took 3m35.058s to build and the trace buffer had 53973
> entries like this one:
>
> bash-1211 [002] ...1 648.238585: __fpu__restore_sig: !NEED_FPU_LOAD, size: 1092, supervisor: 0x0
>
> which means I have
>
> 53973 / (3*60 + 35) =~ 251 XSAVES invocations per second!
>
> And this only during this single workload - I don't even wanna imagine
> what that number would be if it were a huge, overloaded box with a
> signal heavy workload.
>
> And all this overhead to save 16 + 24 bytes supervisor states and throw
> away the rest up to 960 bytes each time.
>
> Err, I don't think so.

This patch serves supervisor states that has not been saved prior to
sigreturn. CET state is in sigcontext and does not need to be saved here.

We can drop this for now, and for new supervisor states, replace
copy_xregs_to_kernel() with a callback that saves only necessary
information.

I will send out v3.

Yu-cheng