Hi, Thomas,
On Fri, Aug 28, 2020 at 5:34 PM Thomas Bogendoerfer
<tsbogend@xxxxxxxxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxxxxx> 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.
In kernel FPU may be enabled temporarily, and it seems a preemptible
kernel may enable FPU for a new process (maybe I'm wrong, this is a
bit complex).
Huacai
>
> Thomas.
>
> --
> Crap can work. Given enough thrust pigs will fly, but it's not necessarily a
> good idea. [ RFC1925, 2.3 ]
