[PATCH 5/5] vfio-pci: Allow to mmap MSI-X table if interrupt remapping is supported
kevin.tian at intel.com
Tue May 3 16:22:15 AEST 2016
> From: Yongji Xie [mailto:xyjxie at 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 at 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?
I think the key whether MSI-X table can be passed through
is related to where hypervisor control is deployed. At least
- 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
- 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?
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