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

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Date:   Thu, 30 Apr 2020 11:41:50 +0100
From:   Will Deacon <will@...nel.org>
To:     Srivatsa Vaddagiri <vatsa@...eaurora.org>
Cc:     konrad.wilk@...cle.com, mst@...hat.com, jasowang@...hat.com,
        jan.kiszka@...mens.com, stefano.stabellini@...inx.com,
        iommu@...ts.linux-foundation.org,
        virtualization@...ts.linux-foundation.org,
        virtio-dev@...ts.oasis-open.org, tsoni@...eaurora.org,
        pratikp@...eaurora.org, christoffer.dall@....com,
        alex.bennee@...aro.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC/PATCH 1/1] virtio: Introduce MMIO ops

On Thu, Apr 30, 2020 at 04:04:46PM +0530, Srivatsa Vaddagiri wrote:
> * Will Deacon <will@...nel.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).

Hmm, but then how would such a kernel work as a guest under all the
spec-compliant hypervisors out there?

> > > +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)?

I haven't looked at that, but I suppose it depends on what your hypervisor
folks are willing to accomodate.

Will