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

From: Alex Bennée
Subject: [Bug 1625216] Re: memory writes via gdb don't work for memory mapped hardware
Date: Tue, 09 Feb 2021 20:52:57 -0000

** Tags added: gdbstub

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

Status in QEMU:

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 

  ;; 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 
       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 
           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) || 
  +            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) {
                                             phys_addr, buf, l);
           } else {
               address_space_rw(cpu->cpu_ases[asidx].as, phys_addr,
  -                             buf, l, 0);
  +                             buf, l, is_write);
           len -= l;
           buf += l;

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