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

[Srivatsa Vaddagiri <vatsa@codeaurora.org>]

* 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)?

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