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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.04.2020 13:01, Paul Durrant wrote:
>> -----Original Message-----
>>>
>>> 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.
>>
> 
> Well the original code IIRC had no justification for setting BME
> and doing it unconditionally does seem dangerous.

I'm not viewing this as dangerous, merely as (typically) pointless.
A well behaved device won't start issuing DMA requests merely
because it had its bus mastering capability enabled. (And in the
context of some IOMMU work of yours you actually stated there are
devices where clearing of this bit won't stop them from doing so.)

> Could we at least make it configurable?
Well, the main question then would be - configurable by which
mechanism?

Jan