git: 11d62b6f31ab - main - linuxkpi: add kernel_fpu_begin/kernel_fpu_end

[Konstantin Belousov]

On Thu, Jan 14, 2021 at 04:23:31AM +0200, Konstantin Belousov wrote:
> On Thu, Jan 14, 2021 at 01:36:27AM +0000, myfreeweb wrote:
> > 
> > 
> > On January 13, 2021 8:58:58 PM UTC, John Baldwin <jhb at FreeBSD.org> wrote:
> > >It doesn't store at all because threads aren't allowed to sleep in a critical
> > >section, so the thread will never give up the CPU while in the FPU section.  If
> > >threads can voluntarily sleep (cv_wait*, *sleep(), etc.) while using
> > >kernel_fpu_begin(), then NOCTX won't work and we will need something else.
> > 
> > Hmm but with no storage at all, how would it work from a syscall?
> > The manpage does mention a "usermode save area" – I was talking about that.
> There is a storage for the user state, always.  When NOCTX is used, FPU state
> is spilled into the _current_ save area, and then kernel lives to the promise
> that the new state after fpu_enter(NOCTX) does not ever need to be saved.
> 
> > 
> > Linux kernel_fpu_begin starts with preempt_disable, so definitely no condvars and the like. No idea about simple time sleeps. But amdgpu doesn't seem to do even that.
> 
> You should get enough assertions fired if something tries to context switch
> while entered NOCTX region.

So this went dead in the mail, and kernel still contains that awful code
that corrupts both userspace and kernel FPU contexts.

I suggest to remove this file at all, since things are not being sorted.