[virtio-dev] Re: [virtio-comment] [PATCH 00/11] Introduce transitional mmr pci device

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Apr 03, 2023 at 10:53:29AM -0400, Parav Pandit wrote:
> 
> 
> On 4/3/2023 10:45 AM, Stefan Hajnoczi wrote:
> > On Fri, Mar 31, 2023 at 01:58:23AM +0300, Parav Pandit wrote:
> > > Overview:
> > > ---------
> > > The Transitional MMR device is a variant of the transitional PCI device.
> > 
> > What does "MMR" mean?
> > 
> memory mapped registers.
> Explained below in the design section and also in relevant patches 6 to 11.
> 
> > > It has its own small Device ID range. It does not have I/O
> > > region BAR; instead it exposes legacy configuration and device
> > > specific registers at an offset in the memory region BAR.
> > > 
> > > Such transitional MMR devices will be used at the scale of
> > > thousands of devices using PCI SR-IOV and/or future scalable
> > > virtualization technology to provide backward
> > > compatibility (for legacy devices) and also future
> > > compatibility with new features.
> > > 
> > > Usecase:
> > > --------
> > > 1. A hypervisor/system needs to provide transitional
> > >     virtio devices to the guest VM at scale of thousands,
> > >     typically, one to eight devices per VM.
> > > 
> > > 2. A hypervisor/system needs to provide such devices using a
> > >     vendor agnostic driver in the hypervisor system.
> > 
> > Is the idea that the hypervisor configures the new Transitional MMR
> > devices and makes them appear like virtio-pci Transitional devices?
> > 
> Yes, but hypervisor is not involved in any configuration parsing or anything
> of that nature.
> It is only a passthrough fowarder from emulated IOBAR to memory mapped
> legacy registers.
> In other words, hypervisor do not care for the registers content at all.

This part I do not see as important. legacy is frozen in time. Implement
it once and you are done. Datapath differences are more important.

> > In other words, the guest doesn't know about Transitional MMR and does
> > not need any code changes.
> > 
> > > 3. A hypervisor system prefers to have single stack regardless of
> > >     virtio device type (net/blk) and be future compatible with a
> > >     single vfio stack using SR-IOV or other scalable device
> > >     virtualization technology to map PCI devices to the guest VM.
> > >     (as transitional or otherwise)
> > 
> > What does this paragraph mean?
> > 
> It means regardless of a VF being transitional MMR VF or 1.x VF without any
> MMR extensions, there is single vfio virtio driver handling both type of
> devices to map to the guest VM.

I don't think this can be vfio. You need a host layer translating
things such as device ID etc.

> > 
> > Modern devices were added to Linux in 2014 and support SR-IOV.
> 
> > Why is it
> > important to support Transitional (which really means Legacy devices,
> > otherwise Modern devices would be sufficient)?
> > 
> To support guest VMs which only understand legacy devices and unfortunately
> they are still in much wider use by the users.

OK but supporting them with a passthrough driver such as vfio does
not seem that important.

-- 
MST


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