Re: [PATCH v6 10/13] vpci/header: reset the command register when adding devices

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.02.2022 15:37, Oleksandr Andrushchenko wrote:
> On 04.02.22 16:30, Jan Beulich wrote:
>> On 04.02.2022 07:34, Oleksandr Andrushchenko wrote:
>>> Reset the command register when assigning a PCI device to a guest:
>>> according to the PCI spec the PCI_COMMAND register is typically all 0's
>>> after reset.
>> It's not entirely clear to me whether setting the hardware register to
>> zero is okay. What wants to be zero is the value the guest observes
>> initially.
> "the PCI spec says the PCI_COMMAND register is typically all 0's after reset."
> Why wouldn't it be ok? What is the exact concern here?

The concern is - as voiced is similar ways before, perhaps in other
contexts - that you need to consider bit-by-bit whether overwriting
with 0 what is currently there is okay. Xen and/or Dom0 may have put
values there which they expect to remain unaltered. I guess
PCI_COMMAND_SERR is a good example: While the guest's view of this
will want to be zero initially, the host having set it to 1 may not
easily be overwritten with 0, or else you'd effectively imply giving
the guest control of the bit.

Jan