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

[Thomas Bogendoerfer <tsbogend@alpha.franken.de>]

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;

Thomas.

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