Re: [PATCH] Allow acpi-tmr size=2

[Michael S. Tsirkin]

On Mon, Jul 13, 2020 at 10:20:12AM +0300, Michael Tokarev wrote:
> 12.07.2020 15:00, Simon John wrote:
> > macos guests no longer boot after commit 
> > 5d971f9e672507210e77d020d89e0e89165c8fc9
> > 
> > acpi-tmr needs 2 byte memory accesses, so breaks as that commit only allows 
> > 4 bytes.
> > 
> > Fixes: 5d971f9e672507210e7 (memory: Revert "memory: accept mismatching 
> > sizes in memory_region_access_valid")
> > Buglink: https://bugs.launchpad.net/qemu/+bug/1886318
> 
> Actually this fixes 77d58b1e47c8d1c661f98f12b47ab519d3561488
> Author: Gerd Hoffmann <kraxel@redhat.com>
> Date:   Thu Nov 22 12:12:30 2012 +0100
> Subject: apci: switch timer to memory api
> Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> 
> because this is the commit which put min_access_size = 4 in there
> (5d971f9e672507210e7 is just a messenger, actual error were here
> earlier but it went unnoticed).
> 
> While min_access_size=4 was most likely an error, I wonder why
> we use 1 now, while the subject says it needs 2? What real min
> size is here for ACPI PM timer?
> 
> /mjt


Well the ACPI spec 1.0b says

4.7.3.3 Power Management Timer (PM_TMR)

...

This register is accessed as 32 bits.

and this text is still there in 6.2.


So it's probably worth it to cite this in the commit log
and explain it's a spec violation.
I think it's better to be restrictive and only allow the
minimal variation from spec - in this case I guess this means 2 byte
reads.

In any case pls do include an explanation for why you picked
one over the other.

> 
> > Signed-off-by: Simon John <git@the-jedi.co.uk>
> > ---
> > Â hw/acpi/core.c | 2 +-
> > Â 1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/hw/acpi/core.c b/hw/acpi/core.c
> > index f6d9ec4f13..05ff29b9d7 100644
> > --- a/hw/acpi/core.c
> > +++ b/hw/acpi/core.c
> > @@ -527,7 +527,7 @@ static void acpi_pm_tmr_write(void *opaque, hwaddr 
> > addr, uint64_t val,
> > Â static const MemoryRegionOps acpi_pm_tmr_ops = {
> > Â Â Â Â  .read = acpi_pm_tmr_read,
> > Â Â Â Â  .write = acpi_pm_tmr_write,
> > -Â Â Â  .valid.min_access_size = 4,
> > +Â Â Â  .valid.min_access_size = 1,
> > Â Â Â Â  .valid.max_access_size = 4,
> > Â Â Â Â  .endianness = DEVICE_LITTLE_ENDIAN,
> > Â };