Re: [RFC PATCH 3/3] hw/openrisc: Add the OpenRISC virtual machine

[Stafford Horne]

On Tue, Jun 07, 2022 at 10:42:08AM +0200, Arnd Bergmann wrote:
> On Tue, Jun 7, 2022 at 10:11 AM Geert Uytterhoeven <geert@linux-m68k.org> 
> wrote:
> > On Sun, Jun 5, 2022 at 9:32 AM Stafford Horne <shorne@gmail.com> wrote:
> > > On Sun, Jun 05, 2022 at 10:58:14AM +0900, Stafford Horne wrote:
> > >     It might be a good idea to revisit the qemu implementation and make
> > >     sure that the extra byteswap is only inserted on m68k and not on
> > >     other targets, but hopefully there are no new targets based on 
> > > goldfish
> > >     anymore and we don't need to care.
> > >
> > > So, it seems that in addition to my patch we would need something in m68k 
> > > to
> > > switch it back to 'native' (big) endian?
> > >
> > > Looking at the m68k kernel/qemu interface I see:
> > >
> > > Pre 5.19:
> > >    (data) <-- kernel(readl / little) <-- m68k qemu (native / big) - 
> > > RTC/PIC
> > >    (data) <-- kernel(__raw_readl / big) <-- m68k qemu (native / big) - TTY
> > >
> > > 5.19:
> > >    (data) <-- kernel(gf_ioread32 / big) <-- m68k qemu (native / big) - all
> > >
> > > The new fixes to add gf_ioread32/gf_iowrite32 fix this for goldfish and 
> > > m68k.
> > > This wouldn't have been an issue for little-endian platforms where 
> > > readl/writel
> > > were originally used.
> > >
> > > Why can't m68k switch to little-endian in qemu and the kernel?  The m68k 
> > > virt
> > > platform is not that old, 1 year? Are there a lot of users that this 
> > > would be a big
> > > problem?
> > >
> > > [1] 
> > > https://lore.kernel.org/lkml/CAK8P3a1oN8NrUjkh2X8jHQbyz42Xo6GSa=5n0gD6vQcXRjmq1Q@mail.gmail.com/
> 
> Goldfish is a very old platform, as far as I know only the kernel port is new.
> I don't know when qemu started shipping goldfish, but changing it now would
> surely break compatibility with whatever OS the port was originally made for.

Hi Arnd,

As far as I can tell goldfish in qemu is not very old. There are 3 devices, 2 
were
added for the m68k virt machine, and 1 for riscv virt.

    $ git lo -- hw/rtc/goldfish_rtc.c
    2022-01-28 2f93d8b04a Peter Maydell    rtc: Move RTC function prototypes to 
their own header 
    2021-03-04 6b9409ba5f Laurent Vivier   goldfish_rtc: re-arm the alarm after 
migration 
    2020-10-13 16b66c5626 Laurent Vivier   goldfish_rtc: change MemoryRegionOps 
endianness to DEVICE_NATIVE_ENDIAN 
    2020-07-22 8380b3a453 Jessica Clarke   goldfish_rtc: Fix non-atomic read 
behaviour of TIME_LOW/TIME_HIGH 
    2020-02-10 9a5b40b842 Anup Patel       hw: rtc: Add Goldfish RTC device 

    $ git lo -- hw/char/goldfish_tty.c
    2021-11-09 65b4c8c759 Philippe Mathieu-Daudé hw/m68k: Fix typo in SPDX tag 
    2021-03-15 8c6df16ff6 Laurent Vivier   hw/char: add goldfish-tty 

    $  git lo -- hw/intc/goldfish_pic.c
    2021-11-09 65b4c8c759 Philippe Mathieu-Daudé hw/m68k: Fix typo in SPDX tag 
    2021-03-15 8785559390 Laurent Vivier   hw/intc: add goldfish-pic 

The mips/loongson3_virt machine now also uses the goldfish_rtc.

The problem with the goldfish device models is that they were defined as
DEVICE_NATIVE_ENDIAN.

    $ grep endianness hw/*/goldfish*
    hw/char/goldfish_tty.c:    .endianness = DEVICE_NATIVE_ENDIAN,
    hw/intc/goldfish_pic.c:    .endianness = DEVICE_NATIVE_ENDIAN,
    hw/rtc/goldfish_rtc.c:    .endianness = DEVICE_NATIVE_ENDIAN,

RISC-V is little-endian so when it was added there was no problem with running
linux goldfish drivers.

MIPS Longson3, added last year, seems to be running as little-endian well, I
understand MIPS can support both big and little endian. However according to
this all Loongson cores are little-endian.

    https://en.wikipedia.org/wiki/Loongson

As I understand when endianness of the devices in qemu are defined as
DEVICE_NATIVE_ENDIAN the device endian takes the endian of the target CPU.
This means that MIPS Loongson3 and RISC-V are affectively running as
little-endian which is what would be expected.

So it appears to me that in qemu that m68k is the only architecture that is
providing goldfish devices on a big-endian architecture.  Also, as far as I
know Linux is the only OS that was updated to cater for that.  If there are
other firmware/bootloaders that expect that maybe they could be updated too?

-Stafford