Re: [PATCH v10 0/7] PCI: Linux kernel SR-IOV support

[Avi Kivity <avi@redhat.com>]

Yang, Sheng wrote:
>>> Patches 2 and 3 of the patchset that enables SR/IOV in kvm [1] suggest
>>> that at the config space is only partially implemented.
>>>
>>> [1] http://thread.gmane.org/gmane.comp.emulators.kvm.devel/29034
>>>       
>> Hi Avi
>>
>> For kernel side, patch 2 is not necessary. Because kernel would read
>> VID/DID directly from pci_dev rather than configuration space, which have
>> been set properly already.
>>
>> And very sorry, for the patch 3. We haven't known exactly what's happened.
>> I think the problem is caused by guest driver, but didn't confirm(and I
>> have some misunderstandings with ZhaoYu for I thought we are agree on the
>> reason, but after confirm with him, he didn't agree). I am doing more
>> investigations to find the real cause.
>>     
>
> Found the reason of patch 3.
>
> After insert guest driver module(vf driver), the driver would do a RMW to the 
> command register to enable Bus Master bit(bit 2). And before that, MMIO bit 
> have been set in the register. But without the patch 3, guest driver won't see 
> the MMIO bit(bit 1), then just set 0x4 to the command register, with the side 
> effect to unmap MMIO in QEmu. So patch 3 is needed(and what I thought before 
> is right).
>
> Unset the bit only affect the QEmu, which would unmap the mapping for MMIO. 
> Kernel side don't need this, so it's OK.
>   

Thanks for the explanations!

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