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

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Date:   Thu, 30 Apr 2020 16:41:56 +0530
From:   Srivatsa Vaddagiri <vatsa@...eaurora.org>
To:     Will Deacon <will@...nel.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

* Will Deacon <will@...nel.org> [2020-04-30 11:41:50]:

> On Thu, Apr 30, 2020 at 04:04:46PM +0530, Srivatsa Vaddagiri wrote:
> > 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?

Ok I see your point and yes for better binary compatibility, the ops have to be
set based on runtime detection of hypervisor capabilities.

> > 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.

I believe ivshmem2_virtio requires hypervisor to support PCI device emulation
(for life-cycle management of VMs), which our hypervisor may not support. A
simple shared memory and doorbell or message-queue based transport will work for
us.

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