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

[Jan Beulich <jbeulich@suse.com>]

On 14.04.2020 13:58, Paul Durrant wrote:
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@suse.com>
>> Sent: 14 April 2020 12:40
>> To: paul@xen.org
>> Cc: 'Shamsundara Havanur, Harsha' <havanur@amazon.com>; xen-devel@lists.xenproject.org;
>> andrew.cooper3@citrix.com; ian.jackson@eu.citrix.com; wl@xen.org; roger.pau@citrix.com
>> Subject: Re: [XEN PATCH v2] hvmloader: Enable MMIO and I/O decode, after all resource allocation
>>
>> 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.)
>>
> 
> It's a line of defence against some devices at least,

What defence? Once we're past hvmloader, the guest can do whatever it
wants anyway.

Jan