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

[Yongji Xie]

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@xxxxxxxxxxxxxxxxxx]
> > > > 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@xxxxxxxxxxxxxxxxxx>
> > > > > 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