Re: [PATCH -next V17 4/7] riscv: entry: Convert to generic entry

["Heiko Stübner" <heiko@sntech.de>]

Am Samstag, 1. April 2023, 16:58:01 CEST schrieb Björn Töpel:
> Björn Töpel <bjorn@kernel.org> writes:
> 
> > (Narrowing the Cc list.)
> >
> > Björn Töpel <bjorn@kernel.org> writes:
> >
> >>>> > > > This has unfortunately broken booting my usual NFS rootfs on both my D1
> >>>> > > > and Icicle. It's one of the Fedora images from David, I think this one:
> >>>> > > > http://fedora.riscv.rocks/kojifiles/work/tasks/3933/1313933/
> >>>> > > >
> >>>> > > > It gets pretty far into things, it's once systemd is operational that
> >>>> > > > things go pear shaped:
> >>>> > >
> >>>> > > Shoulda said, can share the full logs if required of course, but they're
> >>>> > > quite verbose cos systemd etc.
> >>>> >
> >>>> > I was just investigating the same thing just now. So that saves me some
> >>>> > tracking down the culprit :-) .
> >>>> >
> >>>> > My main qemu is living as a "board" in my boardfarm (also doing nfsroot)
> >>>> > as well as my d1 nezha with nfsroot was affected.
> >>>> Can you reproduce it with qemu? Could give me some tips and let me
> >>>> reproduce it on qemu?
> >>
> >> FWIW, I'm getting the systemd issue w/o NFS, on a regular 9p virtfs.
> >>
> >> | $ sudo mmdebstrap --architecture=riscv64 lunar rv-rootfs http://ports.ubuntu.com
> >>
> >> and the rootfs qemu config:
> >>  |  -fsdev local,id=root,path=/path/to/rv-rootfs/,security_model=none \
> >>  |  -device virtio-9p-pci,fsdev=root,mount_tag=/dev/root \
> >>  |  -append "root=/dev/root rw rootfstype=9p rootflags=version=9p2000.L,trans=virtio,cache=mmap,access=any security=none earlycon console=tty0 console=ttyS0"
> >
> > I took a couple of different versions of the series for a spin, and for
> > me it stops working between these versions.
> >
> > $ git fetch https://github.com/guoren83/linux.git generic_entry_v12:ge-v12
> > $ git fetch https://github.com/guoren83/linux.git generic_entry_v13:g2-v13
> >
> > v12 is happy with systemd, v13 is not.
> 
> Hmm, this patch seems to resolve the issue for me:
> 
> --8<--
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index       1f4e37be7eb3..8c258b78c925 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -270,11 +270,11 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>         if (user_mode(regs)) {
>                 ulong syscall = regs->a7;
>  
> -               syscall = syscall_enter_from_user_mode(regs, syscall);
> -
>                 regs->epc += 4;
>                 regs->orig_a0 = regs->a0;
>  
> +               syscall = syscall_enter_from_user_mode(regs, syscall);
> +
>                 if (syscall < NR_syscalls)
>                         syscall_handler(regs, syscall);
>                 else
> -->8--

Woohoo, thanks for tracking that down. This change makes both my
Qemu and D1-Nezha boot again.

> Seems like syscall_enter_from_user_mode() clobber regs->a0.

With a0 also being the return value in the calling conventions (and being
the syscall value here), I guess that could be somehow expected and thus
as your patch does, the a0 value should be saved first I guess?

An it looks like Loonarch also does save a0 before calling
syscall_enter_from_user_mode() [0]

Reviewed-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
Tested-by: Heiko Stuebner <heiko.stuebner@vrull.eu>

Heiko


[0] https://elixir.bootlin.com/linux/latest/source/arch/loongarch/kernel/syscall.c#L54



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