Re: [PATCH] xen: introduce xen_vring_use_dma

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

On Mon, Jun 29, 2020 at 04:46:09PM -0700, Stefano Stabellini wrote:
> > I could imagine some future Xen hosts setting a flag somewhere in the
> > platform capability saying "no xen specific flag, rely on
> > "VIRTIO_F_ACCESS_PLATFORM". Then you set that accordingly in QEMU.
> > How about that?
> 
> Yes, that would be fine and there is no problem implementing something
> like that when we get virtio support in Xen. Today there are still no
> virtio interfaces provided by Xen to ARM guests (no virtio-block/net,
> etc.)
> 
> In fact, in both cases we are discussing virtio is *not* provided by
> Xen; it is a firmware interface to something entirely different:
> 
> 1) virtio is used to talk to a remote AMP processor (RPMesg)
> 2) virtio is used to talk to a secure-world firmware/OS (Trusty)
>
> VIRTIO_F_ACCESS_PLATFORM is not set by Xen in these cases but by RPMesg
> and by Trusty respectively. I don't know if Trusty should or should not
> set VIRTIO_F_ACCESS_PLATFORM, but I think Linux should still work
> without issues.
> 

Any virtio implementation that is not in control of the memory map
(aka not the hypervisor) absolutely must set VIRTIO_F_ACCESS_PLATFORM,
else it is completely broken.

> The xen_domain() check in Linux makes it so that vring_use_dma_api
> returns the opposite value on native Linux compared to Linux as Xen/ARM
> DomU by "accident". By "accident" because there is no architectural
> reason why Linux Xen/ARM DomU should behave differently compared to
> native Linux in this regard.
> 
> I hope that now it is clearer why I think the if (xen_domain()) check
> needs to be improved anyway, even if we fix generic dma_ops with virtio
> interfaces missing VIRTIO_F_ACCESS_PLATFORM.

IMHO that Xen quirk should never have been added in this form..