Date: Wed, 13 Jan 2021 12:58:58 -0800 From: John Baldwin <jhb@FreeBSD.org> To: myfreeweb <greg@unrelenting.technology>, Konstantin Belousov <kostikbel@gmail.com> Cc: 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: <ce860007-4c19-8fb2-05b9-9b9e1bcc0723@FreeBSD.org> In-Reply-To: <A7AF80F3-3E01-44DD-B1FF-49BAEFCF4C4A@unrelenting.technology> References: <202101121143.10CBh02x095972@gitrepo.freebsd.org> <X/2hR9Hi3Jhf5ZNs@kib.kiev.ua> <20210113110826.46fbc900b3c375e7215a8195@bidouilliste.com> <A7AF80F3-3E01-44DD-B1FF-49BAEFCF4C4A@unrelenting.technology>
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On 1/13/21 3:42 AM, myfreeweb wrote: > > > On January 13, 2021 10:08:26 AM UTC, Emmanuel Vadot <manu@bidouilliste.com> wrote: >> On Tue, 12 Jan 2021 15:16:55 +0200 >> Konstantin Belousov <kostikbel@gmail.com> wrote: >> >>> On Tue, Jan 12, 2021 at 11:43:00AM +0000, Emmanuel Vadot wrote: >>>> The branch main has been updated by manu: >>>> >>>> URL: https://cgit.FreeBSD.org/src/commit/?id=11d62b6f31ab4e99df6d0c6c23406b57eaa37f41 >>>> >>>> commit 11d62b6f31ab4e99df6d0c6c23406b57eaa37f41 >>>> Author: Emmanuel Vadot <manu@FreeBSD.org> >>>> AuthorDate: 2021-01-12 11:02:38 +0000 >>>> Commit: Emmanuel Vadot <manu@FreeBSD.org> >>>> CommitDate: 2021-01-12 11:31:00 +0000 >>>> >>>> linuxkpi: add kernel_fpu_begin/kernel_fpu_end >>>> >>>> With newer AMD GPUs (>=Navi,Renoir) there is FPU context usage in the >>>> amdgpu driver. >>>> The `kernel_fpu_begin/end` implementations in drm did not even allow nested >>>> begin-end blocks. >>> >>> Does Linux allow more then one thread to execute kernel_fpu_begin ? >> >> I actually have no idea, adding Greg to cc. > > Looks like they save the context into the current thread state, so yes? (drm doesn't need that) > > Also they seem to do something FPU_KERN_NOCTX like (??) because they disable preemption inside these blocks. > (Where does our NOCTX actually store the state?) 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. However, the code snippet from the stackoverflow URL I posted earlier looks exactly like the NOCTX case where we flush the user FPU state to the thread if the FPU state is "dirty" and then load a clean initial state for use by the FPU. It would also seem to never save the kernel FPU state anywhere by counting on avoiding context switches. So, I think you probably should just make this use NOCTX. -- John Baldwin
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