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

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Message-ID: <87wo2fn1lf.fsf@linaro.org>
Date:   Mon, 03 Aug 2020 17:19:56 +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


Alex Bennée <alex.bennee@...aro.org> writes:

> 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".
<snip>
>
>   - 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.

Just following up after cutting the Xgene and ThunderX PCI bloat from
the kernel the margin is a little smaller:

  -rwxr-xr-x 1 alex alex 83914728 Jul 31 16:39 arm64.nopci/vmlinux*
  -rwxr-xr-x 1 alex alex 85639808 Aug  3 17:12 arm64/vmlinux*

-- 
Alex Bennée