Re: [Qemu-devel] savevm/loadvm

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 10/09/2013 06:47 PM, Paolo Bonzini wrote:
> Il 09/10/2013 09:15, Alexey Kardashevskiy ha scritto:
>> Sorry for my ignorance (I never ever touched this part of qemu) but how can
>> you possibly avoid block.c while doing savevm? The qcow2 driver must not
>> use posix read()/write(), right? So no matter how, all writes end up in
>> bdrv_co_do_writev() which changes blocks number. Or use
>> raw_aio_readv()/raw_aio_writev() API directly? Please give some more hints.
>> Thanks.
> 
> I think Kevin was suggesting using qcow_aio_writev directly, or
> something like that.  But it is not trivial, especially because
> save_vm_state takes byte offsets instead of sectors.  So for now I'd
> still go for the more hacky solution.

I failed to find qcow_aio_writev() or anything like that. qcow2_co_writev()
uses block.c. And I tried this:

diff --git a/block/qcow2.c b/block/qcow2.c
index 4a9888c..17faf8b 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1837,10 +1837,16 @@ static int qcow2_save_vmstate(BlockDriverState *bs,
QEMUIOVector *qiov,
     BDRVQcowState *s = bs->opaque;
     int growable = bs->growable;
     int ret;
+    int64_t total_sectors = bs->total_sectors;

     BLKDBG_EVENT(bs->file, BLKDBG_VMSTATE_SAVE);
     bs->growable = 1;
     ret = bdrv_pwritev(bs, qcow2_vm_state_offset(s) + pos, qiov);
+    /*
+     * Setting @growable may cause underlying bdrv_co_do_writev()
+     * to increase bs->total_sectors and we do not want this to happen.
+     */
+    bs->total_sectors = total_sectors;
     bs->growable = growable;

     return ret;


It breaks loadvm in a different (weird) way, the error is something like
"ram" or "spapr/htab" (streams registered with register_savevm_live())
chunk cannot be read. Need to debug more...


-- 
Alexey