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Re: [Qemu-block] [Qemu-devel] [PATCH 1/1] blk: do not select PFLASH devi

From: Denis V. Lunev
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 1/1] blk: do not select PFLASH device for internal snapshot
Date: Tue, 12 Jan 2016 20:54:53 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.4.0

On 01/12/2016 08:50 PM, Kevin Wolf wrote:
Am 12.01.2016 um 18:40 hat Markus Armbruster geschrieben:
Kevin Wolf <address@hidden> writes:

Am 12.01.2016 um 17:35 hat Denis V. Lunev geschrieben:
On 01/12/2016 06:47 PM, Denis V. Lunev wrote:
On 01/12/2016 06:20 PM, Kevin Wolf wrote:
Am 12.01.2016 um 15:59 hat Paolo Bonzini geschrieben:
On 12/01/2016 15:16, Kevin Wolf wrote:
Thus we should avoid selection of "pflash" drives for VM
state saving.

For now "pflash" is read-write raw image as it configured by libvirt.
Thus there are no such images in the field and we could
safely disable
ability to save state to those images inside QEMU.
This is obviously broken. If you write to the pflash, then it needs to
be snapshotted in order to keep a consistent state.

If you want to avoid snapshotting the image, make it read-only and it
will be skipped even today.
Sort of.  The point of having flash is to _not_ make it read-only, so
that is not a solution.

Flash is already being snapshotted as part of saving RAM state.  In
fact, for this reason the device (at least the one used with OVMF; I
haven't checked other pflash devices) can simply save it back to disk
on the migration destination, without the need to use "migrate -b" or
shared storage.
I don't like very much using IF_PFLASH this way, which is why I hadn't
replied to the patch so far---I hadn't made up my mind about *what* to
suggest instead, or whether to just accept it.  However, it does work.

Perhaps a separate "I know what I am doing" skip-snapshot option?  Or
a device callback saying "not snapshotting this is fine"?
Boy, is this ugly...

What do you do with disk-only snapshots? The recovery only works as long
as you have VM state.

actually I am in a bit of trouble :(

I understand that this is ugly, but I would like to make working
'virsh snapshot' for OVFM VMs. This is necessary for us to make
a release.

Currently libvirt guys generate XML in the following way:

    <type arch='x86_64' machine='pc-i440fx-2.3'>hvm</type>
    <loader readonly='yes'

This results in:

qemu -drive 

This obviously can not pass check in bdrv_all_can_snapshot()
as 'pflash' is RW and raw, i.e. can not be snapshoted.

They have discussed the switch to the following command line:

qemu -drive 

and say that in this case VM state could fall into PFLASH
drive which is should not be big as the location of the
file is different. This means that I am doomed here.

Either we should force libvirt people to forget about their
opinion that pflash should be small which I am unable to
do or I should invent a way to ban VM state saving into

OK. There are 2 options.

1) Ban pflash as it was done.
2) Add 'no-vmstate' flag to -drive (invented just now).

something like this:

diff --git a/block.c b/block.c
index 3e1877d..8900589 100644
--- a/block.c
+++ b/block.c
@@ -881,6 +881,11 @@ static QemuOptsList bdrv_runtime_opts = {
              .help = "Block driver to use for the node",
+            .name = "novmstate",
+            .type = QEMU_OPT_BOOL,
+            .help = "Ignore for selecting to save VM state",
+        },
+        {
              .name = BDRV_OPT_CACHE_WB,
              .type = QEMU_OPT_BOOL,
              .help = "Enable writeback mode",
@@ -957,6 +962,7 @@ static int bdrv_open_common(BlockDriverState
*bs, BdrvChild *file,
      bs->request_alignment = 512;
      bs->zero_beyond_eof = true;
      bs->read_only = !(bs->open_flags & BDRV_O_RDWR);
+    bs->disable_vmstate_save = qemu_opt_get_bool(opts, "novmstate", false);

      if (use_bdrv_whitelist && !bdrv_is_whitelisted(drv, bs->read_only)) {
diff --git a/block/snapshot.c b/block/snapshot.c
index 2d86b88..33cdd86 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -483,6 +483,10 @@ BlockDriverState *bdrv_all_find_vmstate_bs(void)
      while (not_found && (bs = bdrv_next(bs))) {
          AioContext *ctx = bdrv_get_aio_context(bs);

+        if (bs->disable_vmstate_save) {
+            continue;
+        }
          not_found = !bdrv_can_snapshot(bs);
diff --git a/include/block/block_int.h b/include/block/block_int.h
index 256609d..855a209 100644
--- a/include/block/block_int.h
+++ b/include/block/block_int.h
@@ -438,6 +438,9 @@ struct BlockDriverState {
      /* do we need to tell the quest if we have a volatile write cache? */
      int enable_write_cache;

+    /* skip this BDS searching for one to save VM state */
+    bool disable_vmstate_save;
      /* the following member gives a name to every node on the bs graph. */
      char node_name[32];
      /* element of the list of named nodes building the graph */
That sounds like an option. (No pun intended.)

We can discuss the option name (perhaps "vmstate" defaulting to "on" is
better?) and variable names (I'd prefer them to match the option name);
also you'll need to extend the QAPI schema for blockdev-add. But all of
these are minor points and the idea seems sane.
I've always thought that QEMU picking the image to take the VM state is
backwards.  Adding means to guide that pick like "don't pick this one,
please" may help ease the pain, but it's still backwards.

The *user* should pick it.
Designing the API now when it has been in use for ten years is
backwards, too. We have to take it as is and make the best of it.

We could add an optional argument to savevm that tells which image to
save the VM state to. But if it's missing, we still need to make a pick.
Of course, libvirt should then always use that option and then we don't
need a separate vmstate=[on|off] option.

If we go that way, we need to improve loadvm to get VM state from any of
the images of a VM, because the user could have saved the state to any.
(Making that improvement is probably a good idea anyway.)

no. There is window now.

savevm at the moment is implemented in HMP only. There is my pending
patchset to switch to QMP. We can require to do something additional
during that switch.


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