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

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Aug 19, 2013 at 01:15:36PM +0200, Laszlo Ersek wrote:
> 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

Exactly.