Re: [PATCH v5 03/18] pci: isolated address space for PCI bus

[Michael S. Tsirkin]

On Wed, Jan 26, 2022 at 08:07:36PM +0000, Dr. David Alan Gilbert wrote:
> * Stefan Hajnoczi (stefanha@redhat.com) wrote:
> > On Wed, Jan 26, 2022 at 05:27:32AM +0000, Jag Raman wrote:
> > > 
> > > 
> > > > On Jan 25, 2022, at 1:38 PM, Dr. David Alan Gilbert 
> > > > <dgilbert@redhat.com> wrote:
> > > > 
> > > > * Jag Raman (jag.raman@oracle.com) wrote:
> > > >> 
> > > >> 
> > > >>> On Jan 19, 2022, at 7:12 PM, Michael S. Tsirkin <mst@redhat.com> 
> > > >>> wrote:
> > > >>> 
> > > >>> On Wed, Jan 19, 2022 at 04:41:52PM -0500, Jagannathan Raman wrote:
> > > >>>> Allow PCI buses to be part of isolated CPU address spaces. This has a
> > > >>>> niche usage.
> > > >>>> 
> > > >>>> TYPE_REMOTE_MACHINE allows multiple VMs to house their PCI devices in
> > > >>>> the same machine/server. This would cause address space collision as
> > > >>>> well as be a security vulnerability. Having separate address spaces 
> > > >>>> for
> > > >>>> each PCI bus would solve this problem.
> > > >>> 
> > > >>> Fascinating, but I am not sure I understand. any examples?
> > > >> 
> > > >> Hi Michael!
> > > >> 
> > > >> multiprocess QEMU and vfio-user implement a client-server model to 
> > > >> allow
> > > >> out-of-process emulation of devices. The client QEMU, which makes 
> > > >> ioctls
> > > >> to the kernel and runs VCPUs, could attach devices running in a server
> > > >> QEMU. The server QEMU needs access to parts of the client’s RAM to
> > > >> perform DMA.
> > > > 
> > > > Do you ever have the opposite problem? i.e. when an emulated PCI device
> > > 
> > > That’s an interesting question.
> > > 
> > > > exposes a chunk of RAM-like space (frame buffer, or maybe a mapped file)
> > > > that the client can see.  What happens if two emulated devices need to
> > > > access each others emulated address space?
> > > 
> > > In this case, the kernel driver would map the destination’s chunk of 
> > > internal RAM into
> > > the DMA space of the source device. Then the source device could write to 
> > > that
> > > mapped address range, and the IOMMU should direct those writes to the
> > > destination device.
> > > 
> > > I would like to take a closer look at the IOMMU implementation on how to 
> > > achieve
> > > this, and get back to you. I think the IOMMU would handle this. Could you 
> > > please
> > > point me to the IOMMU implementation you have in mind?
> > 
> > I don't know if the current vfio-user client/server patches already
> > implement device-to-device DMA, but the functionality is supported by
> > the vfio-user protocol.
> > 
> > Basically: if the DMA regions lookup inside the vfio-user server fails,
> > fall back to VFIO_USER_DMA_READ/WRITE messages instead.
> > https://github.com/nutanix/libvfio-user/blob/master/docs/vfio-user.rst#vfio-user-dma-read
> > 
> > Here is the flow:
> > 1. The vfio-user server with device A sends a DMA read to QEMU.
> > 2. QEMU finds the MemoryRegion associated with the DMA address and sees
> >    it's a device.
> >    a. If it's emulated inside the QEMU process then the normal
> >       device emulation code kicks in.
> >    b. If it's another vfio-user PCI device then the vfio-user PCI proxy
> >       device forwards the DMA to the second vfio-user server's device B.
> 
> I'm starting to be curious if there's a way to persuade the guest kernel
> to do it for us; in general is there a way to say to PCI devices that
> they can only DMA to the host and not other PCI devices?


But of course - this is how e.g. VFIO protects host PCI devices from
each other when one of them is passed through to a VM.

>  Or that the
> address space of a given PCIe bus is non-overlapping with one of the
> others?



> Dave
> > Stefan
> 
> 
> -- 
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK