linux-kernel - Re: [PATCH v2 0/5] virtio mmio specification enhancement

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Date:   Fri, 31 Jul 2020 16:44:36 +0100
From:   Alex Bennée <alex.bennee@...aro.org>
To:     "Pincus, Josh" <Josh.Pincus@...driver.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "zhabin@...ux.alibaba.com" <zhabin@...ux.alibaba.com>,
        "virtio-dev@...ts.oasis-open.org" <virtio-dev@...ts.oasis-open.org>,
        qemu-devel@...gnu.org
Subject: Re: [PATCH v2 0/5] virtio mmio specification enhancement


Pincus, Josh <Josh.Pincus@...driver.com> writes:

> Hi,
>
>  
>
> We were looking into a similar enhancement for the Virt I/O MMIO transport and came across this project.
>
> This enhancement would be perfect for us.

So there is certainly an interest in optimising MMIO based virtio and
the current read/ack cycle adds additional round trip time for any trap
and emulate hypervisor. However I think there is some resistance to
making MMIO a re-implementation of what PCI already gives us for "free".

I believe the current questions that need to be addressed are:

  - Clear definitions in the spec on doorbells/notifications

    The current virtio spec uses different terms in some places so it
    would be nice to clarify the language and formalise what the
    standard expects from transports w.r.t the capabilities of
    notifications and doorbells.

  - Quantifying the memory foot-print difference between PCI/MMIO

    PCI gives a lot for free including a discovery and IRQ model already
    designed to handle MSI/MSI-X. There is a claim that this brings in a
    lot of bloat but I think there was some debate around the numbers.
    My rough initial experiment with a PCI and non-PCI build with
    otherwise identical VIRTIO configs results in the following:

    16:40:15 c.282% [alex@zen:~/l/l/builds] review/rpmb|… + ls -l arm64/vmlinux arm64.nopci/vmlinux
    -rwxr-xr-x 1 alex alex 83914728 Jul 31 16:39 arm64.nopci/vmlinux*
    -rwxr-xr-x 1 alex alex 86368080 Jul 31 16:33 arm64/vmlinux*

    which certainly implies there could be a fair amount of headroom for
    an MMIO version to implement some features. However I don't know if
    it's fully apples to apples as there maybe unneeded PCI bloat that a
    virtio-only kernel doesn't need.


What are the features you are most interested in?

> Has there been any progress since Feb, 2020?  It looks like the effort
> might have stalled?

I can't speak to the OP's but there is certainly interest from others
that are not the original posters.


-- 
Alex Bennée