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

Yu-cheng Yu
2020-03-04T18:39:34Z

On Mon, 2020-03-02 at 10:09 -0800, Yu-cheng Yu wrote:
> 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.

There is another way to keep this patch...

if (xfeatures_mask_supervisor()) {
fpu->state.xsave.xfeatures &= xfeatures_mask_supervisor();
copy_xregs_to_kernel(&fpu->state.xsave);
}

That way, all the user states are in init state and only supervisor states
are saved, and the buffer format is unchanged.

Yu-cheng