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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.09.2021 11:52, Oleksandr Andrushchenko wrote:
> 
> On 07.09.21 12:19, Jan Beulich wrote:
>> On 07.09.2021 11:07, Oleksandr Andrushchenko wrote:
>>> On 07.09.21 11:49, Jan Beulich wrote:
>>>> On 07.09.2021 10:18, Oleksandr Andrushchenko wrote:
>>>>> So, if we have a hidden PCI device which can be assigned to a guest and 
>>>>> it is literally untouched
>>>>> (not enabled in Dom0) then I think there will be no such reference as 
>>>>> "host assigned values" as
>>>>> most probably the command register will remain in its after reset state.
>>>> What meaning of "hidden" do you imply here? Devices passed to
>>>> pci_{hide,ro}_device() may not be assigned to guests ...
>>> You are completely right here.
>>>> For any other meaning of "hidden", even if the device is completely
>>>> ignored by Dom0,
>>> Dom0less is such a case when a device is assigned to the guest
>>> without Dom0 at all?
>> In this case it is entirely unclear to me what entity it is to have
>> a global view on the PCI subsystem.
>>
>>>>    certain of the properties still cannot be allowed
>>>> to be DomU-controlled.
>>> The list is not that big, could you please name a few you think cannot
>>> be controlled by a guest? I can think of PCI_COMMAND_SPECIAL(?),
>>> PCI_COMMAND_INVALIDATE(?), PCI_COMMAND_PARITY, PCI_COMMAND_WAIT,
>>> PCI_COMMAND_SERR, PCI_COMMAND_INTX_DISABLE which we may want to
>>> be aligned with the "host reference" values, e.g. we only allow those bits
>>> to be set as they are in Dom0.
>> Well, you've compile a list already, and I did say so before as well:
>> Everything except I/O and memory decoding as well as bus mastering
>> needs at least closely looking at. INTX_DISABLE, for example, is
>> something I don't think a guest should be able to directly control.
>> It may still be the case that the host permits it control, but then
>> only indirectly, allowing the host to appropriately adjust its
>> internals.
>>
>> Note that even for I/O and memory decoding as well as bus mastering
>> it may be necessary to limit guest control: In case the host wants
>> to disable any of these (perhaps transiently) despite the guest
>> wanting them enabled.
> 
> Ok, so it is now clear that we need a yet another patch to add a proper
> command register emulation. What is your preference: drop the current
> patch, implement command register emulation and add a "reset patch"
> after that or we can have the patch as is now, but I'll only reset IO/mem and 
> bus
> master bits, e.g. read the real value, mask the wanted bits and write back?

Either order is fine with me as long as the result will be claimed to
be complete until proper emulation is in place.

Jan