Re: [RFC 00/13] vfio: introduce vfio-cxl to support CXL type-2 accelerator passthrough

Jonathan Cameron
2024-10-04T11:40:18Z

> >
> >>
> >> Presumably, the host creates one large CXL region that covers the entire
> >> DPA, while QEMU can virtually partition it into different regions and
> >> map them to different virtual CXL region if QEMU presents multiple HDM
> >> decoders to the guest.
> >
> > I'm not sure why it would do that. Can't think why you'd break up
> > a host region - maybe I'm missing something.
> >
>
> It is mostly concerning about a device can have multiple HDM decoders.
> In the current design, a large physical CXL (pCXL) region with the whole
> DPA will be passed to the userspace. Thinking that the guest will see
> the virtual multiple HDM decoders, which usually SW is asking for, the
> guest SW might create multiple virtual CXL regions. In that case QEMU
> needs to map them into different regions of the pCXL region.

Don't let the guest see multiple HDM decoders?

There is no obvious reason why it would want them other than type
differences.

Why is it useful for a type 2 device to be setup for multiple CXL regions?
It shouldn't be a performance thing. Might be convenient for management
I guess, but the driver can layer it's own allocator etc on top of a single
region so I'm not sure I see a reason to do this...

Jonathan