[PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[Stefano Garzarella]

Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
set to true by default.
To allow the migration, we set this property to false in the hw_compat,
but in the wrong place (hw_compat_4_1).

Since commit 9d7bd0826f was released with QEMU 5.0, we move
'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
will have the pre-patch behavior and the migration can work.

The issue was discovered with vhost-vsock device and 4.2 machine
type without running any kernel in the VM:
    $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
        -device vhost-vsock-pci,guest-cid=4 \
        -monitor stdio -incoming tcp:0:3333

    $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
        -device vhost-vsock-pci,guest-cid=3 \
        -monitor stdio
    (qemu) migrate -d tcp:0:3333

    # qemu-4.2 output
    qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
    qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-vhost_vsock'
    qemu-system-x86_64: load of migration failed: No such file or directory

Reported-by: Jing Zhao <jinzhao@redhat.com>
Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255
Fixes: 9d7bd0826f ("virtio-pci: disable vring processing when bus-mastering is disabled")
Cc: mdroth@linux.vnet.ibm.com
CC: qemu-stable@nongnu.org
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
 hw/core/machine.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index de3b8f1b31..5d6163ab70 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
     { "qxl", "revision", "4" },
     { "qxl-vga", "revision", "4" },
     { "fw_cfg", "acpi-mr-restore", "false" },
+    { "virtio-device", "use-disabled-flag", "false" },
 };
 const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
 
 GlobalProperty hw_compat_4_1[] = {
     { "virtio-pci", "x-pcie-flr-init", "off" },
-    { "virtio-device", "use-disabled-flag", "false" },
 };
 const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);
 
-- 
2.26.2

Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[Stefano Garzarella]

Ping :-)

On Fri, Jan 08, 2021 at 06:12:52PM +0100, Stefano Garzarella wrote:
>Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
>set to true by default.
>To allow the migration, we set this property to false in the hw_compat,
>but in the wrong place (hw_compat_4_1).
>
>Since commit 9d7bd0826f was released with QEMU 5.0, we move
>'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
>will have the pre-patch behavior and the migration can work.
>
>The issue was discovered with vhost-vsock device and 4.2 machine
>type without running any kernel in the VM:
>    $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
>        -device vhost-vsock-pci,guest-cid=4 \
>        -monitor stdio -incoming tcp:0:3333
>
>    $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
>        -device vhost-vsock-pci,guest-cid=3 \
>        -monitor stdio
>    (qemu) migrate -d tcp:0:3333
>
>    # qemu-4.2 output
>    qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
>    qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-vhost_vsock'
>    qemu-system-x86_64: load of migration failed: No such file or directory
>
>Reported-by: Jing Zhao <jinzhao@redhat.com>
>Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255
>Fixes: 9d7bd0826f ("virtio-pci: disable vring processing when bus-mastering is disabled")
>Cc: mdroth@linux.vnet.ibm.com
>CC: qemu-stable@nongnu.org
>Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>---
> hw/core/machine.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
>diff --git a/hw/core/machine.c b/hw/core/machine.c
>index de3b8f1b31..5d6163ab70 100644
>--- a/hw/core/machine.c
>+++ b/hw/core/machine.c
>@@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
>     { "qxl", "revision", "4" },
>     { "qxl-vga", "revision", "4" },
>     { "fw_cfg", "acpi-mr-restore", "false" },
>+    { "virtio-device", "use-disabled-flag", "false" },
> };
> const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
>
> GlobalProperty hw_compat_4_1[] = {
>     { "virtio-pci", "x-pcie-flr-init", "off" },
>-    { "virtio-device", "use-disabled-flag", "false" },
> };
> const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);
>
>-- 
>2.26.2
>
>

Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[Michael S. Tsirkin]

queued, thanks!

On Mon, Jan 18, 2021 at 10:19:55AM +0100, Stefano Garzarella wrote:
> Ping :-)
> 
> On Fri, Jan 08, 2021 at 06:12:52PM +0100, Stefano Garzarella wrote:
> > Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
> > set to true by default.
> > To allow the migration, we set this property to false in the hw_compat,
> > but in the wrong place (hw_compat_4_1).
> > 
> > Since commit 9d7bd0826f was released with QEMU 5.0, we move
> > 'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
> > will have the pre-patch behavior and the migration can work.
> > 
> > The issue was discovered with vhost-vsock device and 4.2 machine
> > type without running any kernel in the VM:
> >    $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
> >        -device vhost-vsock-pci,guest-cid=4 \
> >        -monitor stdio -incoming tcp:0:3333
> > 
> >    $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
> >        -device vhost-vsock-pci,guest-cid=3 \
> >        -monitor stdio
> >    (qemu) migrate -d tcp:0:3333
> > 
> >    # qemu-4.2 output
> >    qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
> >    qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-vhost_vsock'
> >    qemu-system-x86_64: load of migration failed: No such file or directory
> > 
> > Reported-by: Jing Zhao <jinzhao@redhat.com>
> > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255
> > Fixes: 9d7bd0826f ("virtio-pci: disable vring processing when bus-mastering is disabled")
> > Cc: mdroth@linux.vnet.ibm.com
> > CC: qemu-stable@nongnu.org
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > ---
> > hw/core/machine.c | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index de3b8f1b31..5d6163ab70 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
> >     { "qxl", "revision", "4" },
> >     { "qxl-vga", "revision", "4" },
> >     { "fw_cfg", "acpi-mr-restore", "false" },
> > +    { "virtio-device", "use-disabled-flag", "false" },
> > };
> > const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
> > 
> > GlobalProperty hw_compat_4_1[] = {
> >     { "virtio-pci", "x-pcie-flr-init", "off" },
> > -    { "virtio-device", "use-disabled-flag", "false" },
> > };
> > const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);
> > 
> > -- 
> > 2.26.2
> > 
> > 

Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[Dr. David Alan Gilbert]

* Stefano Garzarella (sgarzare@redhat.com) wrote:
> Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
> set to true by default.
> To allow the migration, we set this property to false in the hw_compat,
> but in the wrong place (hw_compat_4_1).
> 
> Since commit 9d7bd0826f was released with QEMU 5.0, we move
> 'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
> will have the pre-patch behavior and the migration can work.

Be a little careful that fixing this probably causes a migration from
5.2->6.0 to fail with this machine type;  so when we do these
type of fixes we often fix an old machine type between some pair of qemu
versions and then break it between a different set.

Dave

> The issue was discovered with vhost-vsock device and 4.2 machine
> type without running any kernel in the VM:
>     $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
>         -device vhost-vsock-pci,guest-cid=4 \
>         -monitor stdio -incoming tcp:0:3333
> 
>     $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
>         -device vhost-vsock-pci,guest-cid=3 \
>         -monitor stdio
>     (qemu) migrate -d tcp:0:3333
> 
>     # qemu-4.2 output
>     qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
>     qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-vhost_vsock'
>     qemu-system-x86_64: load of migration failed: No such file or directory
> 
> Reported-by: Jing Zhao <jinzhao@redhat.com>
> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255
> Fixes: 9d7bd0826f ("virtio-pci: disable vring processing when bus-mastering is disabled")
> Cc: mdroth@linux.vnet.ibm.com
> CC: qemu-stable@nongnu.org
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
>  hw/core/machine.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/hw/core/machine.c b/hw/core/machine.c
> index de3b8f1b31..5d6163ab70 100644
> --- a/hw/core/machine.c
> +++ b/hw/core/machine.c
> @@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
>      { "qxl", "revision", "4" },
>      { "qxl-vga", "revision", "4" },
>      { "fw_cfg", "acpi-mr-restore", "false" },
> +    { "virtio-device", "use-disabled-flag", "false" },
>  };
>  const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
>  
>  GlobalProperty hw_compat_4_1[] = {
>      { "virtio-pci", "x-pcie-flr-init", "off" },
> -    { "virtio-device", "use-disabled-flag", "false" },
>  };
>  const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);
>  
> -- 
> 2.26.2
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[Stefano Garzarella]

On Mon, Jan 18, 2021 at 04:03:12PM +0000, Dr. David Alan Gilbert wrote:
>* Stefano Garzarella (sgarzare@redhat.com) wrote:
>> Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
>> set to true by default.
>> To allow the migration, we set this property to false in the hw_compat,
>> but in the wrong place (hw_compat_4_1).
>>
>> Since commit 9d7bd0826f was released with QEMU 5.0, we move
>> 'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
>> will have the pre-patch behavior and the migration can work.
>
>Be a little careful that fixing this probably causes a migration from
>5.2->6.0 to fail with this machine type;  so when we do these
>type of fixes we often fix an old machine type between some pair of qemu
>versions and then break it between a different set.

Good point!

I did some tests using the example below always using pc-q35-4.2 and it 
seems that works well:

- 5.2 -> 6.0    pass
- 5.2 -> 4.2            FAIL
- 6.0 -> 5.2    pass
- 6.0 -> 4.2    pass
- 4.2 -> 5.2    pass
- 4.2 -> 6.0    pass

Should I run some more tests?

Thanks,
Stefano

>
>Dave
>
>> The issue was discovered with vhost-vsock device and 4.2 machine
>> type without running any kernel in the VM:
>>     $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
>>         -device vhost-vsock-pci,guest-cid=4 \
>>         -monitor stdio -incoming tcp:0:3333
>>
>>     $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
>>         -device vhost-vsock-pci,guest-cid=3 \
>>         -monitor stdio
>>     (qemu) migrate -d tcp:0:3333
>>
>>     # qemu-4.2 output
>>     qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
>>     qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-vhost_vsock'
>>     qemu-system-x86_64: load of migration failed: No such file or directory
>>
>> Reported-by: Jing Zhao <jinzhao@redhat.com>
>> Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255
>> Fixes: 9d7bd0826f ("virtio-pci: disable vring processing when bus-mastering is disabled")
>> Cc: mdroth@linux.vnet.ibm.com
>> CC: qemu-stable@nongnu.org
>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>> ---
>>  hw/core/machine.c | 2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/hw/core/machine.c b/hw/core/machine.c
>> index de3b8f1b31..5d6163ab70 100644
>> --- a/hw/core/machine.c
>> +++ b/hw/core/machine.c
>> @@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
>>      { "qxl", "revision", "4" },
>>      { "qxl-vga", "revision", "4" },
>>      { "fw_cfg", "acpi-mr-restore", "false" },
>> +    { "virtio-device", "use-disabled-flag", "false" },
>>  };
>>  const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
>>
>>  GlobalProperty hw_compat_4_1[] = {
>>      { "virtio-pci", "x-pcie-flr-init", "off" },
>> -    { "virtio-device", "use-disabled-flag", "false" },
>>  };
>>  const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);
>>
>> --
>> 2.26.2
>>
>-- 
>Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
>

Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[Michael S. Tsirkin]

On Tue, Jan 19, 2021 at 09:45:19AM +0100, Stefano Garzarella wrote:
> On Mon, Jan 18, 2021 at 04:03:12PM +0000, Dr. David Alan Gilbert wrote:
> > * Stefano Garzarella (sgarzare@redhat.com) wrote:
> > > Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
> > > set to true by default.
> > > To allow the migration, we set this property to false in the hw_compat,
> > > but in the wrong place (hw_compat_4_1).
> > > 
> > > Since commit 9d7bd0826f was released with QEMU 5.0, we move
> > > 'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
> > > will have the pre-patch behavior and the migration can work.
> > 
> > Be a little careful that fixing this probably causes a migration from
> > 5.2->6.0 to fail with this machine type;  so when we do these
> > type of fixes we often fix an old machine type between some pair of qemu
> > versions and then break it between a different set.
> 
> Good point!
> 
> I did some tests using the example below always using pc-q35-4.2 and it
> seems that works well:
> 
> - 5.2 -> 6.0    pass
> - 5.2 -> 4.2            FAIL
> - 6.0 -> 5.2    pass
> - 6.0 -> 4.2    pass
> - 4.2 -> 5.2    pass
> - 4.2 -> 6.0    pass
> 
> Should I run some more tests?
> 
> Thanks,
> Stefano


dgilbert how about an ack?

> > 
> > Dave
> > 
> > > The issue was discovered with vhost-vsock device and 4.2 machine
> > > type without running any kernel in the VM:
> > >     $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
> > >         -device vhost-vsock-pci,guest-cid=4 \
> > >         -monitor stdio -incoming tcp:0:3333
> > > 
> > >     $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
> > >         -device vhost-vsock-pci,guest-cid=3 \
> > >         -monitor stdio
> > >     (qemu) migrate -d tcp:0:3333
> > > 
> > >     # qemu-4.2 output
> > >     qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
> > >     qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-vhost_vsock'
> > >     qemu-system-x86_64: load of migration failed: No such file or directory
> > > 
> > > Reported-by: Jing Zhao <jinzhao@redhat.com>
> > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255
> > > Fixes: 9d7bd0826f ("virtio-pci: disable vring processing when bus-mastering is disabled")
> > > Cc: mdroth@linux.vnet.ibm.com
> > > CC: qemu-stable@nongnu.org
> > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > ---
> > >  hw/core/machine.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > > index de3b8f1b31..5d6163ab70 100644
> > > --- a/hw/core/machine.c
> > > +++ b/hw/core/machine.c
> > > @@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
> > >      { "qxl", "revision", "4" },
> > >      { "qxl-vga", "revision", "4" },
> > >      { "fw_cfg", "acpi-mr-restore", "false" },
> > > +    { "virtio-device", "use-disabled-flag", "false" },
> > >  };
> > >  const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
> > > 
> > >  GlobalProperty hw_compat_4_1[] = {
> > >      { "virtio-pci", "x-pcie-flr-init", "off" },
> > > -    { "virtio-device", "use-disabled-flag", "false" },
> > >  };
> > >  const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);
> > > 
> > > --
> > > 2.26.2
> > > 
> > -- 
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 

Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[Dr. David Alan Gilbert]

* Stefano Garzarella (sgarzare@redhat.com) wrote:
> On Mon, Jan 18, 2021 at 04:03:12PM +0000, Dr. David Alan Gilbert wrote:
> > * Stefano Garzarella (sgarzare@redhat.com) wrote:
> > > Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
> > > set to true by default.
> > > To allow the migration, we set this property to false in the hw_compat,
> > > but in the wrong place (hw_compat_4_1).
> > > 
> > > Since commit 9d7bd0826f was released with QEMU 5.0, we move
> > > 'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
> > > will have the pre-patch behavior and the migration can work.
> > 
> > Be a little careful that fixing this probably causes a migration from
> > 5.2->6.0 to fail with this machine type;  so when we do these
> > type of fixes we often fix an old machine type between some pair of qemu
> > versions and then break it between a different set.
> 
> Good point!
> 
> I did some tests using the example below always using pc-q35-4.2 and it
> seems that works well:
> 
> - 5.2 -> 6.0    pass
> - 5.2 -> 4.2            FAIL
> - 6.0 -> 5.2    pass
> - 6.0 -> 4.2    pass
> - 4.2 -> 5.2    pass
> - 4.2 -> 6.0    pass
> 
> Should I run some more tests?

Apologies for the delay; I had to step back and understand a bit about
what was going on.

The problem here is that you're sending a 'disabled' subsection when
that option is true; your patch doesn't change the 4.1 machine type but
it does change the 4.2 machine type; and that makes the 4.2 machine type
not send it; so that means your patched version *will* work to existing
code (because it's a subsection anyway it doesn't break the stream
format when it's missing).


Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>

> Thanks,
> Stefano
> 
> > 
> > Dave
> > 
> > > The issue was discovered with vhost-vsock device and 4.2 machine
> > > type without running any kernel in the VM:
> > >     $ qemu-4.2 -M pc-q35-4.2,accel=kvm \
> > >         -device vhost-vsock-pci,guest-cid=4 \
> > >         -monitor stdio -incoming tcp:0:3333
> > > 
> > >     $ qemu-5.2 -M pc-q35-4.2,accel=kvm \
> > >         -device vhost-vsock-pci,guest-cid=3 \
> > >         -monitor stdio
> > >     (qemu) migrate -d tcp:0:3333
> > > 
> > >     # qemu-4.2 output
> > >     qemu-system-x86_64: Failed to load virtio-vhost_vsock:virtio
> > >     qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:03.0/virtio-vhost_vsock'
> > >     qemu-system-x86_64: load of migration failed: No such file or directory
> > > 
> > > Reported-by: Jing Zhao <jinzhao@redhat.com>
> > > Buglink: https://bugzilla.redhat.com/show_bug.cgi?id=1907255
> > > Fixes: 9d7bd0826f ("virtio-pci: disable vring processing when bus-mastering is disabled")
> > > Cc: mdroth@linux.vnet.ibm.com
> > > CC: qemu-stable@nongnu.org
> > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > ---
> > >  hw/core/machine.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > > index de3b8f1b31..5d6163ab70 100644
> > > --- a/hw/core/machine.c
> > > +++ b/hw/core/machine.c
> > > @@ -70,12 +70,12 @@ GlobalProperty hw_compat_4_2[] = {
> > >      { "qxl", "revision", "4" },
> > >      { "qxl-vga", "revision", "4" },
> > >      { "fw_cfg", "acpi-mr-restore", "false" },
> > > +    { "virtio-device", "use-disabled-flag", "false" },
> > >  };
> > >  const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
> > > 
> > >  GlobalProperty hw_compat_4_1[] = {
> > >      { "virtio-pci", "x-pcie-flr-init", "off" },
> > > -    { "virtio-device", "use-disabled-flag", "false" },
> > >  };
> > >  const size_t hw_compat_4_1_len = G_N_ELEMENTS(hw_compat_4_1);
> > > 
> > > --
> > > 2.26.2
> > > 
> > -- 
> > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> > 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

Re: [PATCH] virtio: move 'use-disabled-flag' property to hw_compat_4_2

[Stefano Garzarella]

On Wed, Jan 27, 2021 at 02:28:13PM +0000, Dr. David Alan Gilbert wrote:
>* Stefano Garzarella (sgarzare@redhat.com) wrote:
>> On Mon, Jan 18, 2021 at 04:03:12PM +0000, Dr. David Alan Gilbert wrote:
>> > * Stefano Garzarella (sgarzare@redhat.com) wrote:
>> > > Commit 9d7bd0826f introduced a new 'use-disabled-flag' property
>> > > set to true by default.
>> > > To allow the migration, we set this property to false in the hw_compat,
>> > > but in the wrong place (hw_compat_4_1).
>> > >
>> > > Since commit 9d7bd0826f was released with QEMU 5.0, we move
>> > > 'use-disabled-flag' property to hw_compat_4_2, so 4.2 machine types
>> > > will have the pre-patch behavior and the migration can work.
>> >
>> > Be a little careful that fixing this probably causes a migration from
>> > 5.2->6.0 to fail with this machine type;  so when we do these
>> > type of fixes we often fix an old machine type between some pair of qemu
>> > versions and then break it between a different set.
>>
>> Good point!
>>
>> I did some tests using the example below always using pc-q35-4.2 and it
>> seems that works well:
>>
>> - 5.2 -> 6.0    pass
>> - 5.2 -> 4.2            FAIL
>> - 6.0 -> 5.2    pass
>> - 6.0 -> 4.2    pass
>> - 4.2 -> 5.2    pass
>> - 4.2 -> 6.0    pass
>>
>> Should I run some more tests?
>
>Apologies for the delay; I had to step back and understand a bit about
>what was going on.

no problem :-)

>
>The problem here is that you're sending a 'disabled' subsection when
>that option is true; your patch doesn't change the 4.1 machine type but
>it does change the 4.2 machine type; and that makes the 4.2 machine type
>not send it; so that means your patched version *will* work to existing
>code (because it's a subsection anyway it doesn't break the stream
>format when it's missing).
>

Thanks for the detailed explanation!

>
>Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
>

Thanks,
Stefano