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

[Parav Pandit <parav@nvidia.com>]

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.

> 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.

> 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.