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

[Laszlo Ersek <lersek@redhat.com>]

(resending, with Paolo's address corrected)

On 06/01/15 22:31, Gabriel L. Somlo wrote:
> On Mon, Jun 01, 2015 at 02:00:22PM -0400, Gabriel L. Somlo wrote:
>> 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 :)
> 
> Actually, it's much worse than that. A lot of the data items stored in
> fw_cfg are just pointers to somewhere in the qemu process address
> space, and I have no confidence that these pointers are guaranteed to
> make sense in the address space of the *destination* qemu process...
> 
> I guess the only reason this isn't a problem is that nobody currently
> attempts to access fw_cfg after a migration ? :)

I thought once fw_cfg was rebased to an (anonymous?) RAMBlock footing,
migration became a non-issue. But, admittedly, I haven't thought much
about it.

Laszlo