Re: Deprecate the ppc405 boards in QEMU?

[Christophe Leroy]

Le 21/10/2021 à 09:25, Thomas Huth a écrit :
> On 21/10/2021 08.48, Christophe Leroy wrote:
> > Le 20/10/2021 à 15:16, Christophe Leroy a écrit :
> > > Le 20/10/2021 à 14:43, Cédric Le Goater a écrit :
> > > > On 10/20/21 13:42, BALATON Zoltan wrote:
> > > > > On Wed, 20 Oct 2021, Philippe Mathieu-Daudé wrote:
> > > > > > On 10/5/21 14:29, Thomas Huth wrote:
> > > > > > > On 05/10/2021 14.20, BALATON Zoltan wrote:
> > > > > > > > On Tue, 5 Oct 2021, Cédric Le Goater wrote:
> > > > > > > > > On 10/5/21 08:18, Alexey Kardashevskiy wrote:
> > > > > > > > > > On 05/10/2021 15:44, Christophe Leroy wrote:
> > > > > > > > > > > Le 05/10/2021 à 02:48, David Gibson a écrit :
> > > > > > > > > > > > On Fri, Oct 01, 2021 at 04:18:49PM +0200, Thomas Huth wrote:
> > > > > > > > > > > > > On 01/10/2021 15.04, Christophe Leroy wrote:
> > > > > > > > > > > > > > Le 01/10/2021 à 14:04, Thomas Huth a écrit :
> > > > > > > > > > > > > > > On 01/10/2021 13.12, Peter Maydell wrote:
> > > > > > > > > > > > > > > > On Fri, 1 Oct 2021 at 10:43, Thomas Huth <thuth@redhat.com>
> > > > > > > > > > > > > > > > wrote:
> > > > > > > > > > > > > > > > > Nevertheless, as long as nobody has a hint where to 
> > > > > > > > > > > > > > > > > find that
> > > > > > > > > > > > > > > > > ppc405_rom.bin, I think both boards are pretty useless in
> > > > > > > > > > > > > > > > > QEMU (as far as I
> > > > > > > > > > > > > > > > > can see, they do not work without the bios at all, so it's
> > > > > > > > > > > > > > > > > also not possible
> > > > > > > > > > > > > > > > > to use a Linux image with the "-kernel" CLI option 
> > > > > > > > > > > > > > > > > directly).
> > > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > It is at least in theory possible to run bare-metal code on
> > > > > > > > > > > > > > > > either board, by passing either a pflash or a bios 
> > > > > > > > > > > > > > > > argument.
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > True. I did some more research, and seems like there was 
> > > > > > > > > > > > > > > once
> > > > > > > > > > > > > > > support for those boards in u-boot, but it got removed 
> > > > > > > > > > > > > > > there a
> > > > > > > > > > > > > > > couple of years ago already:
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > https://gitlab.com/qemu-project/u-boot/-/commit/98f705c9cefdf 
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > https://gitlab.com/qemu-project/u-boot/-/commit/b147ff2f37d5b 
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > https://gitlab.com/qemu-project/u-boot/-/commit/7514037bcdc37 
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > > But I agree that there seem to be no signs of anybody 
> > > > > > > > > > > > > > > > actually
> > > > > > > > > > > > > > > > successfully using these boards for anything, so we should
> > > > > > > > > > > > > > > > deprecate-and-delete them.
> > > > > > > > > > > > > > >
> > > > > > > > > > > > > > > Yes, let's mark them as deprecated now ... if someone 
> > > > > > > > > > > > > > > still uses
> > > > > > > > > > > > > > > them and speaks up, we can still revert the deprecation 
> > > > > > > > > > > > > > > again.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > I really would like to be able to use them to validate 
> > > > > > > > > > > > > > Linux Kernel
> > > > > > > > > > > > > > changes, hence looking for that missing BIOS.
> > > > > > > > > > > > > >
> > > > > > > > > > > > > > If we remove ppc405 from QEMU, we won't be able to do any
> > > > > > > > > > > > > > regression
> > > > > > > > > > > > > > tests of Linux Kernel on those processors.
> > > > > > > > > > > > >
> > > > > > > > > > > > > If you/someone managed to compile an old version of u-boot for
> > > > > > > > > > > > > one of these
> > > > > > > > > > > > > two boards, so that we would finally have something for
> > > > > > > > > > > > > regression testing,
> > > > > > > > > > > > > we can of course also keep the boards in QEMU...
> > > > > > > > > > > >
> > > > > > > > > > > > I can see that it would be usefor for some cases, but unless 
> > > > > > > > > > > > someone
> > > > > > > > > > > > volunteers to track down the necessary firmware and look 
> > > > > > > > > > > > after it, I
> > > > > > > > > > > > think we do need to deprecate it - I certainly don't have the
> > > > > > > > > > > > capacity
> > > > > > > > > > > > to look into this.
> > > > > > > > > > >
> > > > > > > > > > > I will look at it, please allow me a few weeks though.
> > > > > > > > > >
> > > > > > > > > > Well, building it was not hard but now I'd like to know what 
> > > > > > > > > > board
> > > > > > > > > > QEMU actually emulates, there are way too many codenames and 
> > > > > > > > > > PVRs.
> > > > > > > > >
> > > > > > > > > yes. We should try to reduce the list below. Deprecating embedded
> > > > > > > > > machines
> > > > > > > > > is one way.
> > > > > > > >
> > > > > > > > Why should we reduce that list? It's good to have different cpu
> > > > > > > > options when one wants to test code for different PPC versions 
> > > > > > > > (maybe
> > > > > > > > also in user mode) or just to have a quick list of these at one 
> > > > > > > > place.
> > > > > > >
> > > > > > > I think there are many CPUs in that list which cannot be used 
> > > > > > > with any
> > > > > > > board, some of them might be also in a very incomplete state. So
> > > > > > > presenting such a big list to the users is confusing and might 
> > > > > > > create
> > > > > > > wrong expectations. It would be good to remove at least the CPUs 
> > > > > > > which
> > > > > > > are really completely useless.
> > > > > >
> > > > > > Maybe only remove some from system emulation but keep all of them
> > > > > > in user emulation?
> > > > >
> > > > > Or keep them all but mark those that are not tested/may be 
> > > > > incomplete? So the used can see what is expected to work and what 
> > > > > may need to be fixed. That way somebody may try and fix it whereas 
> > > > > if it's not there they are unlikely to try to add it.
> > > >
> > > >
> > > > The bamboo machine with 440 CPUs is booting with the latest kernel
> > > > and we have an acceptance test for it now, thanks to Thomas. There
> > > > is not much effort in keeping them in a working state until someone
> > > > volunteers. Hopefully, Christophe is making sure that we are not
> > > > breaking anything with Linux support.
> > > >
> > > > The 405 machine are still close to deprecation I think. We are still
> > > > struggling to boot one with mainline Linux, using uboot provided by
> > > > Thomas which skips SDRAM init. It is not clear to me if u-boot is
> > > > strictly necessary. It depends if Linux relies on it to do some
> > > > pre-initialization of HW. I guess once we find a good DTS for it, or
> > > > not, we can take a decision.
> > >
> > > I now have a hacked configuration booting linux with the hotfoot DTS 
> > > and the kernel is booting up to starting /init
> > >
> > > Then it is faulting forever taking a DSI for write protection. The 
> > > problem is now likely in Linux and I'm investigating it, but I'm 
> > > having problem with GDB (7.8.1), I'm hitting the bug 
> > > https://sourceware.org/bugzilla/show_bug.cgi?id=17700
> > >
> > > And GDB 11.1 coredumps while reading vmlinux's symbols
> > >
> > > Hopefully I'll find a GDB version between 7.8 and 11 that works.
> >
> > I bisected the issue to 
> > https://github.com/linuxppc/linux/commit/52ae92cc290f0506eef9ad5466bb453ce4a9e80e 
>
>
> You could argue that this commit introduced a bug, or at least a bad 
> behavior in the kernel: According to the 405 user's manual that I have:
>
>   10.2 Reserved Fields
>   For all registers having fields marked as reserved, the reserved
>   fields should be written as zero and read as undefined. That is,
>   when writing to a reseved field, write a 0 to the field. When
>   reading from a reserved field, ignore the field.
>
> Thus the kernel should not write non-zero bits into the upper bits of 
> this register.

Yes, SHOULD, not SHALL ...


The following changes fixes the issue in the kernel though:

diff --git a/arch/powerpc/kernel/head_40x.S b/arch/powerpc/kernel/head_40x.S
index 618d7f3f9c95..bcde7e284bfa 100644
--- a/arch/powerpc/kernel/head_40x.S
+++ b/arch/powerpc/kernel/head_40x.S
@@ -317,8 +317,9 @@ _ENTRY(saved_ksp_limit)
        /* The bailout.  Restore registers to pre-exception conditions
         * and call the heavyweights to help us out.
         */
-       mtspr   SPRN_PID, r12
        mtcrf   0x80, r12
+       rlwinm  r12, r12, 0, 0xff
+       mtspr   SPRN_PID, r12
        mfspr   r9, SPRN_SPRG_SCRATCH4
        mfspr   r12, SPRN_SPRG_SCRATCH3
        mfspr   r11, SPRN_SPRG_SCRATCH6
@@ -397,8 +398,9 @@ _ENTRY(saved_ksp_limit)
        /* The bailout.  Restore registers to pre-exception conditions
         * and call the heavyweights to help us out.
         */
-       mtspr   SPRN_PID, r12
        mtcrf   0x80, r12
+       rlwinm  r12, r12, 0, 0xff
+       mtspr   SPRN_PID, r12
        mfspr   r9, SPRN_SPRG_SCRATCH4
        mfspr   r12, SPRN_SPRG_SCRATCH3
        mfspr   r11, SPRN_SPRG_SCRATCH6
@@ -542,8 +544,9 @@ finish_tlb_load:

        /* Done...restore registers and get out of here.
        */
-       mtspr   SPRN_PID, r12
        mtcrf   0x80, r12
+       rlwinm  r12, r12, 0, 0xff
+       mtspr   SPRN_PID, r12
        mfspr   r9, SPRN_SPRG_SCRATCH4
        mfspr   r12, SPRN_SPRG_SCRATCH3
        mfspr   r11, SPRN_SPRG_SCRATCH6


> > The problem is in QEMU, it should ignore upper bits of PID register.
>
> Agreed, that's certainly necessary, too.

Yes it seems clear in 405 user's manual chapter 7 Memory Management, 
especially in all drawings, that only bits 24:31 are taken into account 
from PID register.


Christophe