Re: [PATCH 5/5] vfio-pci: Allow to mmap MSI-X table if interrupt remapping is supported

[Yongji Xie <xyjxie@linux.vnet.ibm.com>]

On 2016/5/5 17:36, Tian, Kevin wrote:

>> From: Yongji Xie
>> Sent: Tuesday, May 03, 2016 3:34 PM
>>
>> On 2016/5/3 14:22, Tian, Kevin wrote:
>>
>>>> From: Yongji Xie [mailto:xyjxie@linux.vnet.ibm.com]
>>>> Sent: Tuesday, May 03, 2016 2:08 PM
>>>>
>>>> On 2016/5/3 13:34, Tian, Kevin wrote:
>>>>
>>>>>> From: Yongji Xie
>>>>>> Sent: Wednesday, April 27, 2016 8:43 PM
>>>>>>
>>>>>> This patch enables mmapping MSI-X tables if hardware supports
>>>>>> interrupt remapping which can ensure that a given pci device
>>>>>> can only shoot the MSIs assigned for it.
>>>>>>
>>>>>> With MSI-X table mmapped, we also need to expose the
>>>>>> read/write interface which will be used to access MSI-X table.
>>>>>>
>>>>>> Signed-off-by: Yongji Xie <xyjxie@linux.vnet.ibm.com>
>>>>> A curious question here. Does "allow to mmap MSI-X" essentially
>>>>> mean that KVM guest can directly read/write physical MSI-X
>>>>> structure then?
>>>>>
>>>>> Thanks
>>>>> Kevin
>>>>>
>>>> Here we just allow to mmap MSI-X table in kernel. It doesn't
>>>> mean all KVM guest can directly read/write physical MSI-X
>>>> structure. This should be decided by QEMU. For PPC64
>>>> platform, we would allow to passthrough the MSI-X table
>>>> because we know guest kernel would not write physical
>>>> MSI-X structure when enabling MSI.
>>>>
>>> A bit confused here. If guest kernel doesn't need to write
>>> physical MSI-X structure, what's the point of passing through
>>> the table then?
>> We want to allow the MSI-X table because there may be
>> some critical registers in the same page as the MSI-X table.
>> We have to handle the mmio access to these register in QEMU
>> rather than in guest if mmapping MSI-X table is disallowed.
> So you mean critical registers in same MMIO BAR as MSI-X
> table, instead of two MMIO BARs in same page (the latter I
> suppose with your whole patchset it won't happen then)?

Yes. That's what I mean!

Thanks,
Yongji