Re: [PATCH v1 1/1] hw/pci: Disable PCI_ERR_UNCOR_MASK register for machine type < 8.0

[Juan Quintela <quintela@redhat.com>]

Fiona Ebner <f.ebner@proxmox.com> wrote:
> Am 03.05.23 um 02:27 schrieb Leonardo Bras:
>> Since it's implementation on v8.0.0-rc0, having the PCI_ERR_UNCOR_MASK
>> set for machine types < 8.0 will cause migration to fail if the target
>> QEMU version is < 8.0.0 :
>> 
>> qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x10a read: 40 device: 0 cmask: ff wmask: 0 w1cmask:0
>> qemu-system-x86_64: Failed to load PCIDevice:config
>> qemu-system-x86_64: Failed to load e1000e:parent_obj
>> qemu-system-x86_64: error while loading state for instance 0x0 of device '0000:00:02.0/e1000e'
>> qemu-system-x86_64: load of migration failed: Invalid argument
>> 
>> The above test migrated a 7.2 machine type from QEMU master to QEMU 7.2.0,
>> with this cmdline:
>> 
>> ./qemu-system-x86_64 -M pc-q35-7.2 [-incoming XXX]
>> 
>> In order to fix this, property x-pcie-err-unc-mask was introduced to
>> control when PCI_ERR_UNCOR_MASK is enabled. This property is enabled by
>> default, but is disabled if machine type <= 7.2.
>> 
>> Fixes: 010746ae1d ("hw/pci/aer: Implement PCI_ERR_UNCOR_MASK register")
>> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
>> Signed-off-by: Leonardo Bras <leobras@redhat.com>
>
> Thank you for the patch!
>
> Closes: https://gitlab.com/qemu-project/qemu/-/issues/1576
>
> AFAICT, this breaks (forward) migration from 8.0 to 8.0 + this patch
> when using machine type <= 7.2. That is because after this patch, when
> using machine type <= 7.2, the wmask for the register is not set and
> when 8.0 sends a nonzero value for the register, the error condition in
> get_pci_config_device() will trigger again.

I think that works correctly.
See https://lists.gnu.org/archive/html/qemu-devel/2023-05/msg02733.html

What we have (before this patch) (using abbrevs as in the doc before)

Current state:

(1) qemu-8.0 -M pc-8.0 -> qemu-8.0 -M pc-8.0 works

    not affected by the patch

(2) qemu-7.2 -M pc-7.2 -> qemu-8.0 -M pc-8.0 works

    works well because 7.2 don't change that field

(3) qemu-8.0 -M pc-7.2 -> qemu-7.2 -M pc-7.2 fails

With the patch we fixed 3, so once it is in stable, 1 and 2 continue as
usual and for (3) we will have:

(3) qemu-8.0.1 -M pc-7.2 -> qemu-7.2 -M pc-7.2 works

If what you mean is that:

(3) qemu-8.0 -M pc-7.2 -> qemu-8.0.1 -M pc-7.2 works

Will fail, that is true, but I can think a "sane" way to fix this.


> Is it necessary to also handle that? Maybe by special casing the error
> condition in get_pci_config_device() to be prepared to accept such a
> stream from 8.0?

Well, we can do that, but it is to the pci maintainers to decide if that
is "sane".


> If that is considered not worth it, consider this:
>
> Tested-by: Fiona Ebner <f.ebner@proxmox.com>
>
> Best Regards,
> Fiona

Later, Juan.