Re: ACPI endianness

[Jonathan Cameron]

On Mon, 11 Oct 2021 08:19:01 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Mon, Oct 11, 2021 at 12:13:55PM +0200, BALATON Zoltan wrote:
> > On Mon, 11 Oct 2021, Philippe Mathieu-Daudé wrote:  
> > > On 10/10/21 15:24, BALATON Zoltan wrote:  
> > > > Hello,
> > > > 
> > > > I'm trying to fix shutdown and reboot on pegasos2 which uses ACPI as
> > > > part of the VIA VT8231 (similar to and modelled in hw/isa/vt82c686b.c)
> > > > and found that the guest writes to ACPI PM1aCNT register come out with
> > > > wrong endianness but not shure why. I have this:
> > > > 
> > > > $ qemu-system-ppc -M pegasos2 -monitor stdio
> > > > (qemu) info mtree
> > > > [...]
> > > > memory-region: pci1-io
> > > >   0000000000000000-000000000000ffff (prio 0, i/o): pci1-io
> > > > [...]
> > > >     0000000000000f00-0000000000000f7f (prio 0, i/o): via-pm
> > > >       0000000000000f00-0000000000000f03 (prio 0, i/o): acpi-evt
> > > >       0000000000000f04-0000000000000f05 (prio 0, i/o): acpi-cnt
> > > >       0000000000000f08-0000000000000f0b (prio 0, i/o): acpi-tmr
> > > > 
> > > > memory-region: system
> > > >   0000000000000000-ffffffffffffffff (prio 0, i/o): system
> > > >     0000000000000000-000000001fffffff (prio 0, ram): pegasos2.ram
> > > >     0000000080000000-00000000bfffffff (prio 0, i/o): alias pci1-mem0-win
> > > > @pci1-mem 0000000080000000-00000000bfffffff
> > > >     00000000c0000000-00000000dfffffff (prio 0, i/o): alias pci0-mem0-win
> > > > @pci0-mem 00000000c0000000-00000000dfffffff
> > > >     00000000f1000000-00000000f100ffff (prio 0, i/o): mv64361
> > > >     00000000f8000000-00000000f8ffffff (prio 0, i/o): alias pci0-io-win
> > > > @pci0-io 0000000000000000-0000000000ffffff
> > > >     00000000f9000000-00000000f9ffffff (prio 0, i/o): alias pci0-mem1-win
> > > > @pci0-mem 0000000000000000-0000000000ffffff
> > > >     00000000fd000000-00000000fdffffff (prio 0, i/o): alias pci1-mem1-win
> > > > @pci1-mem 0000000000000000-0000000000ffffff
> > > >     00000000fe000000-00000000feffffff (prio 0, i/o): alias pci1-io-win
> > > > @pci1-io 0000000000000000-0000000000ffffff
> > > >     00000000ff800000-00000000ffffffff (prio 0, i/o): alias pci1-mem3-win
> > > > @pci1-mem 00000000ff800000-00000000ffffffff
> > > >     00000000fff00000-00000000fff7ffff (prio 0, rom): pegasos2.rom
> > > > 
> > > > The guest (which is big endian PPC and I think wotks on real hardware)
> > > > writes to 0xf05 in the io region which should be the high byte of the
> > > > little endian register but in the acpi code it comes out wrong, instead
> > > > of 0x2800 I get in acpi_pm1_cnt_write: val=0x28  
> > > 
> > > Looks like a northbridge issue (MV64340).
> > > Does Pegasos2 enables bus swapping?
> > > See hw/pci-host/mv64361.c:585:
> > > 
> > > static void warn_swap_bit(uint64_t val)
> > > {
> > >    if ((val & 0x3000000ULL) >> 24 != 1) {
> > >        qemu_log_mask(LOG_UNIMP, "%s: Data swap not implemented", 
> > > __func__);
> > >    }
> > > }  
> > 
> > No, guests don't use this feature just byteswap themselves and write little
> > endian values to the mapped io region. (Or in this case just write one byte
> > of the 16 bit value, it specifically writes 0x28 to 0xf05.) That's why I
> > think the device model should not byteswap itself so the acpi regions should
> > be declared native endian and let the guest handle it. Some mentions I've
> > found say that native endian means don't byteswap, little endian means
> > byteswap if vcpu is big endian and big endian means byteswap if vcpu is
> > little endian. Since guests already account for this and write little endian
> > values to these regions there's no need to byteswap in device model and
> > changing to native endian should not affect little endian machines so unless
> > there's a better argument I'll try sending a patch.
> > 
> > Regards,
> > BALATON Zoltan  
> 
> native endian means endian-ness is open-coded in the device handler
> itself.  I think you just found a bug in memory core.
> 
> static const MemoryRegionOps acpi_pm_cnt_ops = {
>     .read = acpi_pm_cnt_read,
>     .write = acpi_pm_cnt_write,
>     .impl.min_access_size = 2,
>     .valid.min_access_size = 1,
>     .valid.max_access_size = 2,
>     .endianness = DEVICE_LITTLE_ENDIAN,
> };
> 
> 
> Because of that     .impl.min_access_size = 2,
> the 1 byte write should be converted to a 2 byte
> read+2 byte write.
> 
> docs/devel/memory.rst just says:
> - .impl.min_access_size, .impl.max_access_size define the access sizes
>   (in bytes) supported by the *implementation*; other access sizes will be
>   emulated using the ones available.  For example a 4-byte write will be
>   emulated using four 1-byte writes, if .impl.max_access_size = 1.
> 
> 
> 
> Instead what we have is:
> 
> MemTxResult memory_region_dispatch_write(MemoryRegion *mr,
>                                          hwaddr addr,
>                                          uint64_t data,
>                                          MemOp op,
>                                          MemTxAttrs attrs)
> {
>     unsigned size = memop_size(op);
> 
>     if (!memory_region_access_valid(mr, addr, size, true, attrs)) {
>         unassigned_mem_write(mr, addr, data, size);
>         return MEMTX_DECODE_ERROR;
>     }
> 
>     adjust_endianness(mr, &data, op);
> 
> 
> ---
> 
> 
> static void adjust_endianness(MemoryRegion *mr, uint64_t *data, MemOp op)
> {
>     if ((op & MO_BSWAP) != devend_memop(mr->ops->endianness)) {
>         switch (op & MO_SIZE) {
>         case MO_8:
>             break;
>         case MO_16:
>             *data = bswap16(*data);
>             break;
>         case MO_32:
>             *data = bswap32(*data);
>             break;
>         case MO_64:
>             *data = bswap64(*data);
>             break;
>         default:
>             g_assert_not_reached();
>         }
>     }
> }
> 
> so the byte swap is based on size before extending it to
> .impl.min_access_size, not after.
> 
> Also, no read happens which I suspect is also a bug,
> but could be harmless ...

The lack of read modify write looks like this again:
https://lore.kernel.org/qemu-devel/20210513124737.00002b2d@Huawei.com/

byte swap part is new to me though...

> 
> Paolo, any feedback here?
>