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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.09.2021 14:42, Oleksandr Andrushchenko wrote:
> On 09.09.21 14:53, Jan Beulich wrote:
>> 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.
> Agree
>>   Which will want the bit set at least as
>> long as either MSI or MSI-X is enabled for the device.
> But what is the INTx relation to MSI/MSI-X here?

Devices are not supposed to signal interrupts two different ways at a
time. They may enable only one - pin based, MSI, or MSI-X.

Jan