Date: Thu, 22 Apr 2021 13:19:07 +0300 From: Konstantin Belousov <kostikbel@gmail.com> To: myfreeweb <greg@unrelenting.technology> Cc: John Baldwin <jhb@freebsd.org>, Emmanuel Vadot <manu@freebsd.org>, src-committers@freebsd.org, dev-commits-src-all@freebsd.org, dev-commits-src-main@freebsd.org Subject: Re: git: 11d62b6f31ab - main - linuxkpi: add kernel_fpu_begin/kernel_fpu_end Message-ID: <YIFNm8S3oFFrJNTM@kib.kiev.ua> In-Reply-To: <X/%2BrI2QGxMOANgpL@kib.kiev.ua> References: <202101121143.10CBh02x095972@gitrepo.freebsd.org> <X/2hR9Hi3Jhf5ZNs@kib.kiev.ua> <20210113110826.46fbc900b3c375e7215a8195@bidouilliste.com> <A7AF80F3-3E01-44DD-B1FF-49BAEFCF4C4A@unrelenting.technology> <ce860007-4c19-8fb2-05b9-9b9e1bcc0723@FreeBSD.org> <171B7072-9BAE-46BB-82BA-4792AEBAD0EB@unrelenting.technology> <X/%2BrI2QGxMOANgpL@kib.kiev.ua>
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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@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.
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