Re: [Qemu-devel] [PATCH v3 2/2] loader: put FW CFG ROM files into RAM

[Laszlo Ersek <lersek@redhat.com>]

On 08/19/13 13:06, Laszlo Ersek wrote:
> On 08/13/13 00:43, Michael S. Tsirkin wrote:

>> @@ -646,6 +669,7 @@ int rom_add_file(const char *file, const char *fw_dir,
>>      if (rom->fw_file && fw_cfg) {
>>          const char *basename;
>>          char fw_file_name[56];
>> +        void *data;
>>  
>>          basename = strrchr(rom->fw_file, '/');
>>          if (basename) {
>> @@ -655,8 +679,15 @@ int rom_add_file(const char *file, const char *fw_dir,
>>          }
>>          snprintf(fw_file_name, sizeof(fw_file_name), "%s/%s", rom->fw_dir,
>>                   basename);
>> -        fw_cfg_add_file(fw_cfg, fw_file_name, rom->data, rom->romsize);
>>          snprintf(devpath, sizeof(devpath), "/rom@%s", fw_file_name);
>> +
>> +        if (rom_file_in_ram) {
>> +            data = rom_set_mr(rom, OBJECT(fw_cfg), devpath);
>> +        } else {
>> +            data = rom->data;
>> +        }
>> +
>> +        fw_cfg_add_file(fw_cfg, fw_file_name, data, rom->romsize);
> 
> This seems OK, but if "rom_file_in_ram" is nonzero, then we'll store the
> ROM contents in the qemu process twice -- once in "rom->data" (allocated
> just a bit higher up, not shown in context), and in the new RAMBlock.
> 
> This is no bug of course, I'm just wondering if we could drop/repoint
> "rom->data" in this case.
> 
>>      } else {
>>          snprintf(devpath, sizeof(devpath), "/rom@" TARGET_FMT_plx, addr);
>>      }
>> @@ -731,7 +762,12 @@ static void rom_reset(void *unused)
>>          if (rom->data == NULL) {
>>              continue;
>>          }
>> -        cpu_physical_memory_write_rom(rom->addr, rom->data, rom->datasize);
>> +        if (rom->mr) {
>> +            void *host = memory_region_get_ram_ptr(rom->mr);
>> +            memcpy(host, rom->data, rom->datasize);
>> +        } else {
>> +            cpu_physical_memory_write_rom(rom->addr, rom->data, rom->datasize);
>> +        }
> 
> Hmmm. Why is this (ie. the pre-patch resetting) necessary at all?
> 
> Is this due to the writeability of fw_cfg files via the ioport
> (fw_cfg_write())? I think that modifies "rom->data" unconditionally
> (which is currently kept separate from the RAMBlock, see above).
> 
> So, regarding the patched version:
> - not sure if the RAMBlock can change at all -- it is neither mapped
> into guest-phys address space, nor does fw_cfg_write() touch it,
> - *if* the guest modifies the contents under "rom->addr", via
> fw_cfg_write(), then the hva-space memcpy() is insufficient.

Sorry, I'm wrong here. The patched rom_add_file() ensures that
fw_cfg_write() modifies the correct backing store. Also, we need to keep
"rom->data" around even if "rom_file_in_ram" is set, because that's
where we restore the RAMBlock contents from, in case of a reset.

Laszlo