Re: [PATCH 8/9] vpci/header: Reset the command register when adding devices

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.09.2021 13:48, Oleksandr Andrushchenko wrote:
> On 09.09.21 12:21, Jan Beulich wrote:
>> For the bit in question, where the goal appears to be to have hardware
>> hold the OR of guest and host values, an approach similar to that used
>> for some of the MSI / MSI-X bits might be chosen: Maintain guest and
>> host bits in software, and update hardware (at least) when the
>> effective resulting value changes. A complicating fact here is, though,
>> that unlike for the MSI / MSI-X bits here Dom0 (pciback or its PCI
>> susbstem) may also have a view on what the setting ought to be.
> 
> The bigger question here is what can we take as the reference for INTx
> bit, e.g. if Dom0 didn't enable/configured the device being passed through
> than its COMMAND register may still be in after reset state and IMO there is
> no guarantee it has the values we can say are "as host wants them"

In the absence of Dom0 controlling the device, I think we ought to take
Xen's view as the "host" one. Which will want the bit set at least as
long as either MSI or MSI-X is enabled for the device.

Jan