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

[Yongji Xie]

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.

> 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

Acutually, we are not aimed at accessing MSI-X table from
guest. So I think it's safe to passthrough MSI-X table if we
can make sure guest kernel would not touch MSI-X table in
normal code path such as para-virtualized guest kernel on PPC64.

Thanks,
Yongji