Re: [PATCH] physmem: allow debug writes to MMIO regions

[Perry Hung]

Philippe, Peter,

Thank you for the comments. I am not even sure what the semantics of 
putting a breakpoint or watchpoint
on device regions are supposed to be. I would imagine it is 
architecture-specific as to whether this is even allowed.

It appears for example, that armv8-a allows watchpoints to be set on any 
type of memory. armv7-a prohibits
watchpoints on Device or Strongly-ordered memory that might be accessed 
by instructions multiple times
(e.g LDM and LDC instructions).

What is the current behavior for QEMU and what should 
breakpoints/watchpoints do when placed on IO memory?

-perry

On 5/20/24 10:22 AM, Peter Maydell wrote:
> On Wed, 15 May 2024 at 13:49, Philippe Mathieu-Daudé <philmd@linaro.org> wrote:
> > Hi Perry,
> >
> > On 14/5/24 01:33, Perry Hung wrote:
> > > Writes from GDB to memory-mapped IO regions are currently silently
> > > dropped. cpu_memory_rw_debug() calls address_space_write_rom(), which
> > > calls address_space_write_rom_internal(), which ignores all non-ram/rom
> > > regions.
> > >
> > > Add a check for MMIO regions and direct those to address_space_rw()
> > > instead.
> > Reported-by: Andreas Rasmusson <andreas.rasmusson@gmail.com>
> > BugLink: https://bugs.launchpad.net/qemu/+bug/1625216
> >
> > > Resolves: https://gitlab.com/qemu-project/qemu/-/issues/213
> > > Signed-off-by: Perry Hung <perry@mosi.io>
> > > ---
> > >    system/physmem.c | 5 ++++-
> > >    1 file changed, 4 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/system/physmem.c b/system/physmem.c
> > > index 342b7a8fd4..013cdd2ab1 100644
> > > --- a/system/physmem.c
> > > +++ b/system/physmem.c
> > > @@ -3508,7 +3508,10 @@ int cpu_memory_rw_debug(CPUState *cpu, vaddr addr,
> > >            if (l > len)
> > >                l = len;
> > >            phys_addr += (addr & ~TARGET_PAGE_MASK);
> > > -        if (is_write) {
> > > +        if (cpu_physical_memory_is_io(phys_addr)) {
> > > +            res = address_space_rw(cpu->cpu_ases[asidx].as, phys_addr, attrs,
> > > +                                   buf, l, is_write);
> > > +        } else if (is_write) {
> > >                res = address_space_write_rom(cpu->cpu_ases[asidx].as, phys_addr,
> > >                                              attrs, buf, l);
> > >            } else {
> The other option is to make address_space_write_rom_internal()
> also write to devices...
>
> > I wonder if we shouldn't be safer with a preliminary patch
> > adding a 'can_do_io' boolean argument to cpu_memory_rw_debug()
> > (updating the call sites), then this patch would become:
> >
> >       if (can_do_io && cpu_physical_memory_is_io(phys_addr)) {
> >
> > One of my worries for example is if someone accidently insert
> > a breakpoint at a I/O address, the device might change its
> > state and return MEMTX_OK which is confusing.
> You can definitely do some silly things if we remove this
> restriction.
>
> On the other hand if you're using gdb as a debugger on real
> (bare metal) hardware does anything stop you doing that?
>
> -- PMM