Re: [Qemu-devel] fw cfg files cross-version migration races

[Gabriel L. Somlo]

On Mon, Jun 01, 2015 at 05:44:47PM +0200, Michael S. Tsirkin wrote:
> > > Shouldn't we migrate the fw cfg data that the source host generates
> > > originally,  rather than trying to play games make sure the way it
> > > is re-generated on dest doesn't change.
> > 
> > Right now, in hw/nvram/fw_cfg.c, we have:
> > 
> > struct FWCfgState {
> >     /*< private >*/
> >     SysBusDevice parent_obj; 
> >     /*< public >*/
> >     
> >     FWCfgEntry entries[2][FW_CFG_MAX_ENTRY];
> >     FWCfgFiles *files;
> >     uint16_t cur_entry;
> >     uint32_t cur_offset;
> >     Notifier machine_ready;
> > };
> > 
> > and, later:
> > 
> > static const VMStateDescription vmstate_fw_cfg = {
> >     .name = "fw_cfg",
> >     .version_id = 2,
> >     .minimum_version_id = 1,
> >     .fields = (VMStateField[]) {
> >         VMSTATE_UINT16(cur_entry, FWCfgState),
> >         VMSTATE_UINT16_HACK(cur_offset, FWCfgState, is_version_1),
> >         VMSTATE_UINT32_V(cur_offset, FWCfgState, 2),
> >         VMSTATE_END_OF_LIST() 
> >     }
> > };  
> > 
> > Would this be as simple as adding a VMSTATE_ARRAY* for 'entries'
> > and something like a VMSTATE_VBUFFER_ALLOC_UINT32 for 'files', which
> > is dynamically allocated the first time a fwcfg "file" is inserted ?
> > 
> > The one catch is that the value of the "files" pointer is itself a
> > fw_cfg entry (FW_CFG_FILE_DIR), so that would need to be "patched"
> > on the destination side...
> > 
> > I do like the idea of simply migrating the full content of the fw_cfg
> > device though, seems like the safest solution.
> > 
> > Thanks much,
> > --Gabriel
> 
> OK but you need to do a bunch of work on load, e.g. some fw cfg
> entries trigger callbacks on access, etc.

Oh, you mean here:

  typedef struct FWCfgEntry {
    uint32_t len;
    uint8_t *data;
    void *callback_opaque;
    FWCfgReadCallback read_callback;
  } FWCfgEntry;

... I can't just assume that 'read_callback' is a valid function
pointer in the context of the destination host ?

Ouch, that could get painful really really quickly :)

Thanks,
--Gabriel