[Bug 1625216] Re: memory writes via gdb don't work for memory mapped hardware

[Peter Maydell]

The code has moved around somewhat, but it's still true that writes by
gdb don't go to devices -- cpu_memory_rw_debug() calls
address_space_write_rom() which calls address_space_write_rom_internal()
which simply skips writing for non-ram/rom regions.

I'm not sure if the gdb accesses should be special cased or if we should
just make address_space_write_rom() write to devices (which would also
affect eg ELF file loading, which is useful in some odd corner cases).


** Changed in: qemu
       Status: New => Confirmed

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https://bugs.launchpad.net/bugs/1625216

Title:
  memory writes via gdb don't work for memory mapped hardware

Status in QEMU:
  Confirmed

Bug description:
  When I remote-debug a qemu-guest and attempt to write to a memory mapped 
location, the
  write-handler for the concerned device will not be called. All 
write-requiests from
  gdb are delegated to cpu_physical_memory_write_rom(...). a function that 
writes to the 
  underlying ram-block.

  I believe requests to memory mapped hardware should be delegated to 
  address_space_rw(). 

  example:
  ;; a memory mapped device. No effect, the write-handler is not called
  (gdb) set *0xfff3c000 = 48

  ;; a ram or rom-block. Thos works. The value is changed.
  (gdb) set *0x100000 = 48

  
  ----------------------------------------

  Here's my suggested patch. As noted in the comment, it could perhaps be
  improved for the (rare) case when the write-request from gdb spans multiple 
  memory regions.

  $ git diff   85bc2a15121e8bcd9f15eb75794a1eacca9d84bd HEAD ../exec.c
  diff --git a/exec.c b/exec.c
  index c4f9036..45ef896 100644
  --- a/exec.c
  +++ b/exec.c
  @@ -3676,6 +3676,7 @@ int cpu_memory_rw_debug(CPUState *cpu, target_ulong 
addr,
       int l;
       hwaddr phys_addr;
       target_ulong page;
  +    bool is_memcpy_access;
   
       while (len > 0) {
           int asidx;
  @@ -3691,13 +3692,32 @@ int cpu_memory_rw_debug(CPUState *cpu, target_ulong 
addr,
           if (l > len)
               l = len;
           phys_addr += (addr & ~TARGET_PAGE_MASK);
  +
           if (is_write) {
  +            /* if ram/rom region we access the memory 
  +               via memcpy instead of via the cpu */
  +            hwaddr mr_len, addr1;
  +            AddressSpace *as = cpu->cpu_ases[asidx].as;
  +            MemoryRegion *mr = address_space_translate(as, phys_addr, 
&addr1, &mr_len, is_write);
  +            is_memcpy_access  = memory_region_is_ram(mr) || 
memory_region_is_romd(mr);
  +            if(mr_len < len) {
  +                /* TODO, mimic more of the loop over mr chunks as 
  +                   done in cpu_physical_memory_write_internal */ 
  +                printf("warning: we dont know whether all bytes "
  +                       "to be written are ram/rom or io\n");
  +            }
  +        }
  +        else {
  +            is_memcpy_access = false;
  +        }
  +        
  +        if (is_write && is_memcpy_access) {
               cpu_physical_memory_write_rom(cpu->cpu_ases[asidx].as,
                                             phys_addr, buf, l);
           } else {
               address_space_rw(cpu->cpu_ases[asidx].as, phys_addr,
                                MEMTXATTRS_UNSPECIFIED,
  -                             buf, l, 0);
  +                             buf, l, is_write);
           }
           len -= l;
           buf += l;

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