Re: [PATCH v4 02/11] vpci: cancel pending map/unmap on vpci removal

[Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>]

On 17.11.21 10:28, Jan Beulich wrote:
> On 05.11.2021 07:56, Oleksandr Andrushchenko wrote:
>> From: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
>>
>> When a vPCI is removed for a PCI device it is possible that we have
>> scheduled a delayed work for map/unmap operations for that device.
>> For example, the following scenario can illustrate the problem:
>>
>> pci_physdev_op
>>     pci_add_device
>>         init_bars -> modify_bars -> defer_map -> raise_softirq(SCHEDULE_SOFTIRQ)
>>     iommu_add_device <- FAILS
>>     vpci_remove_device -> xfree(pdev->vpci)
>>
>> leave_hypervisor_to_guest
>>     vpci_process_pending: v->vpci.mem != NULL; v->vpci.pdev->vpci == NULL
>>
>> For the hardware domain we continue execution as the worse that
>> could happen is that MMIO mappings are left in place when the
>> device has been deassigned
>>
>> For unprivileged domains that get a failure in the middle of a vPCI
>> {un}map operation we need to destroy them, as we don't know in which
>> state the p2m is. This can only happen in vpci_process_pending for
>> DomUs as they won't be allowed to call pci_add_device.
>>
>> Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
> Thinking about it some more, I'm not convinced any of this is really
> needed in the presented form.
The intention of this patch was to handle error conditions which are
abnormal, e.g. when iommu_add_device fails and we are in the middle
of initialization. So, I am trying to cancel all pending work which might
already be there and not to crash.
>   Removal of a vPCI device is the analogue
> of hot-unplug on baremetal. That's not a "behind the backs of
> everything" operation. Instead the host admin has to prepare the
> device for removal, which will result in it being quiescent (which in
> particular means no BAR adjustments anymore). The act of removing the
> device from the system has as its virtual counterpart "xl pci-detach".
> I think it ought to be in this context when pending requests get
> drained, and an indicator be set that no further changes to that
> device are permitted. This would mean invoking from
> vpci_deassign_device() as added by patch 4, not from
> vpci_remove_device(). This would yield removal of a device from the
> host being independent of removal of a device from a guest.
>
> The need for vpci_remove_device() seems questionable in the first
> place: Even for hot-unplug on the host it may be better to require a
> pci-detach from (PVH) Dom0 before the actual device removal. This
> would involve an adjustment to the de-assignment logic for the case
> of no quarantining: We'd need to make sure explicit de-assignment
> from Dom0 actually removes the device from there; right now
> de-assignment assumes "from DomU" and "to Dom0 or DomIO" (depending
> on quarantining mode).
Please see above. What you wrote might be perfectly fine for
the "expected" removals, but what about the errors which are
out of administrator's control?
>
> Thoughts?
>
> Jan
>
Thank you,
Oleksandr