Re: [XEN PATCH v2] hvmloader: Enable MMIO and I/O decode, after all resource allocation

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.04.2020 11:22, Shamsundara Havanur, Harsha wrote:
> On Tue, 2020-04-14 at 11:14 +0200, Jan Beulich wrote:
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>>
>>
>> On 14.04.2020 11:00, Shamsundara Havanur, Harsha wrote:
>>> On Tue, 2020-04-14 at 09:42 +0200, Jan Beulich wrote:
>>>> On 13.04.2020 23:33, Harsha Shamsundara Havanur wrote:
>>>>> @@ -289,9 +293,22 @@ void pci_setup(void)
>>>>>                     devfn>>3, devfn&7, 'A'+pin-1, isa_irq);
>>>>>          }
>>>>>
>>>>> -        /* Enable bus mastering. */
>>>>> +        /*
>>>>> +         * Disable bus mastering, memory and I/O space, which
>>>>> is
>>>>> typical device
>>>>> +         * reset state. It is recommended that BAR programming
>>>>> be
>>>>> done whilst
>>>>> +         * decode bits are cleared to avoid incorrect mappings
>>>>> being created,
>>>>> +         * when 64-bit memory BAR is programmed first by
>>>>> writing
>>>>> the lower half
>>>>> +         * and then the upper half, which first maps to an
>>>>> address
>>>>> under 4G
>>>>> +         * replacing any RAM mapped in that address, which is
>>>>> not
>>>>> restored
>>>>> +         * back after the upper half is written and PCI memory
>>>>> is
>>>>> correctly
>>>>> +         * mapped to its intended high mem address.
>>>>> +         *
>>>>> +         * Capture the state of bus master to restore it back
>>>>> once
>>>>> BAR
>>>>> +         * programming is completed.
>>>>> +         */
>>>>>          cmd = pci_readw(devfn, PCI_COMMAND);
>>>>> -        cmd |= PCI_COMMAND_MASTER;
>>>>> +        pci_devfn_decode_type[devfn] = cmd &
>>>>> ~(PCI_COMMAND_MEMORY
>>>>>> PCI_COMMAND_IO);
>>>>>
>>>>> +        cmd &= ~(PCI_COMMAND_MASTER | PCI_COMMAND_MEMORY |
>>>>> PCI_COMMAND_IO);
>>>>
>>>> The disabling of MASTER was put under question in v1 already.
>>>
>>> Disabling of MASTER is done whilst programming BARs and it is
>>> restored
>>> back to its previous value in the loop at the end of pci_setup
>>> function.
>>
>> Yet didn't Andrew indicate he knows of devices which get upset if
>> MASTER _ever_ gets cleared?
> 
> Previous commit enabled MASTER for all functions. I am bit confused
> here on the consensus on enabling/disabling/retaining BME.
> Should we even care about MASTER?

With the commit introducing its universal setting, I'm afraid to
avoid regressions we can't sensibly alter the behavior unless it
can be explained clearly why the original change must have been
outright wrong.

Jan