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

[Tian, Kevin]

> 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?

I think the key whether MSI-X table can be passed through
is related to where hypervisor control is deployed. At least
for x86:

- When irq remapping is not enabled, host/hypervisor needs
to control physical interrupt message including vector/dest/etc.
directly in MSI-X structure, so we cannot allow a guest to 
access it;

- when irq remapping is enabled, host/hypervisor can control
interrupt routing in irq remapping table. However MSI-X
also needs to be configured as remappable format. In this
manner we also cannot allow direct access from guest.

The only sane case to pass through MSI-X structure, is a
mechanism similar to irq remapping but w/o need to change
original MSI-X format so direct access from guest side is
safe. Is it the case in PPC64?

Thanks
Kevin