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

From: Huacai Chen <chenhc@lemote.com>
To: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
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: Wed, 2 Sep 2020 14:54:10 +0800	[thread overview]
Message-ID: <CAAhV-H4dZMgjdHiLrebBz10J7asadErM53F+TM=fLV_66dhiLw@mail.gmail.com> (raw)
In-Reply-To: <20200826124646.GA9809@alpha.franken.de>

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;
I tried this way but it doesn't work, the reason is that the resume
routine in r4k_switch.S save the current hardware status into
THREAD_STATUS, but CU2 in hardware is cleared in its caller (i.e.,
switch_to). However, I will send V5 to use ST0_KERNEL_CUMASK in all
possible places to avoid #ifdefs.

Huacai

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