Re: [PATCH V3 1/2] MIPS: Loongson-3: Enable COP2 usage in kernel

From: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
To: Huacai Chen <chenhc@lemote.com>
Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>,
	"open list:MIPS" <linux-mips@vger.kernel.org>,
	Fuxin Zhang <zhangfx@lemote.com>,
	Zhangjin Wu <wuzhangjin@gmail.com>
Subject: Re: [PATCH V3 1/2] MIPS: Loongson-3: Enable COP2 usage in kernel
Date: Fri, 28 Aug 2020 11:33:08 +0200	[thread overview]
Message-ID: <20200828093308.GA7888@alpha.franken.de> (raw)
In-Reply-To: <CAAhV-H4_=zeYZ4HSnLmqSk_utNJ=daS_s+UQdvx7n=iEKVW2pQ@mail.gmail.com>

On Fri, Aug 28, 2020 at 05:21:19PM +0800, Huacai Chen wrote:
> Hi, Thomas,
> 
> On Fri, Aug 28, 2020 at 4:43 PM Thomas Bogendoerfer
> <tsbogend@alpha.franken.de> wrote:
> >
> > On Thu, Aug 27, 2020 at 11:48:29AM +0800, Huacai Chen wrote:
> > > Hi, Thomas,
> > >
> > > On Wed, Aug 26, 2020 at 8:48 PM Thomas Bogendoerfer
> > > <tsbogend@alpha.franken.de> wrote:
> > > >
> > > > On Fri, Aug 14, 2020 at 05:44:18PM +0800, Huacai Chen wrote:
> > > > > On Tue, Aug 11, 2020 at 8:08 PM Thomas Bogendoerfer
> > > > > <tsbogend@alpha.franken.de> wrote:
> > > > > > this comes with it's own memcpy/memset and stuff, I don't see a reason why
> > > > > > COP2 needs to be enabled there,
> > > > > gslq/gssq can also be generated by toolchains.
> > > >
> > > > I don't want to introduce every single CPU optimization bits into such
> > > > a closed first stage loader. So please use $(filter-out) in
> > > > arch/mips/boot/compressed/Makefile to disable creation of 16byte load/stores.
> > > >
> > > > > > which is correct for all user space process, otherwise the whole
> > > > > > cop2 exception thing wouldn't work. And if cop2 exception handling
> > > > > > has been run it's set in THREAD_STATUS.
> > > > > >
> > > > > THREAD_STATUS means thread_struct.cp0_status, which is the cp0_status
> > > > > when a process runs in kernel-space. KSTK_STATUS (what you have seen
> > > > > in copy_thread_tls() below) means cp0_status in a process's kernel
> > > > > stack, which saves the cp0_status when a process runs in user-space.
> > > > > Whether COP2 exception can work depends on that KSTK_STATUS (but not
> > > > > THREAD_STATUS) should not contain CU2 at the first time. So, whether
> > > > > or not THREAD_STATUS contains CU2, it won't break COP2 handling.
> > > >
> > > > so why don't we fix the the in-kernel cp0_status instead ?
> > > >
> > > > How about this ?
> > > >
> > > > diff --git a/arch/mips/kernel/process.c b/arch/mips/kernel/process.c
> > > > index 90b869297893..26fb77a8d406 100644
> > > > --- a/arch/mips/kernel/process.c
> > > > +++ b/arch/mips/kernel/process.c
> > > > @@ -133,6 +133,7 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long usp,
> > > >         /*  Put the stack after the struct pt_regs.  */
> > > >         childksp = (unsigned long) childregs;
> > > >         p->thread.cp0_status = read_c0_status() & ~(ST0_CU2|ST0_CU1);
> > > > +       p->thread.cp0_status |= ST0_KERNEL_CUMASK;
> > > >         if (unlikely(p->flags & PF_KTHREAD)) {
> > > >                 /* kernel thread */
> > > >                 unsigned long status = p->thread.cp0_status;
> > > This seems a good idea, I will send a new version.
> >
> > IMHO it migt be even better to just use
> >
> > p->thread.cp0_status = read_c0_status();
> >
> > without masking. We are in kernel, so we took care of whatever CU1/CU2
> > handling had to be done at kernel entry. So keeping the current cp0 status
> > for the new thread looks more sane to me.
> I think this may cause FPU be enabled in kernel by mistake.

if it is enabled at that point, it was already enabled in kernel.

Thomas.

-- 
Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
good idea.                                                [ RFC1925, 2.3 ]