Re: [RFC/PATCH 1/1] virtio: Introduce MMIO ops

Srivatsa Vaddagiri
2020-04-30T10:34:56Z

* Will Deacon <will@kernel.org> [2020-04-30 11:14:32]:

> > +#ifdef CONFIG_VIRTIO_MMIO_OPS
> >
> > +static struct virtio_mmio_ops *mmio_ops;
> > +
> > +#define virtio_readb(a) mmio_ops->mmio_readl((a))
> > +#define virtio_readw(a) mmio_ops->mmio_readl((a))
> > +#define virtio_readl(a) mmio_ops->mmio_readl((a))
> > +#define virtio_writeb(val, a) mmio_ops->mmio_writeb((val), (a))
> > +#define virtio_writew(val, a) mmio_ops->mmio_writew((val), (a))
> > +#define virtio_writel(val, a) mmio_ops->mmio_writel((val), (a))
>
> How exactly are these ops hooked up? I'm envisaging something like:
>
> ops = spec_compliant_ops;
> [...]
> if (firmware_says_hypervisor_is_buggy())
> ops = magic_qcom_ops;
>
> am I wrong?

If CONFIG_VIRTIO_MMIO_OPS is defined, then I expect this to be unconditionally
set to 'magic_qcom_ops' that uses hypervisor-supported interface for IO (for
example: message_queue_send() and message_queue_recevie() hypercalls).

> > +int register_virtio_mmio_ops(struct virtio_mmio_ops *ops)
> > +{
> > + pr_info("Registered %s as mmio ops\n", ops->name);
> > + mmio_ops = ops;
>
> Not looking good, and really defeats the point of standardising this stuff
> imo.

Ok. I guess the other option is to standardize on a new virtio transport (like
ivshmem2-virtio)?