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

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Message-ID: <20200211160541.GA37446@chaop.bj.intel.com>
Date:   Tue, 11 Feb 2020 16:05:41 +0000
From:   Chao Peng <chao.p.peng@...ux.intel.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Zha Bin <zhabin@...ux.alibaba.com>, linux-kernel@...r.kernel.org,
        jasowang@...hat.com, slp@...hat.com,
        virtio-dev@...ts.oasis-open.org, qemu-devel@...gnu.org,
        gerry@...ux.alibaba.com, jing2.liu@...ux.intel.com
Subject: Re: [PATCH v2 0/5] virtio mmio specification enhancement

On Mon, Feb 10, 2020 at 06:44:50AM -0500, Michael S. Tsirkin wrote:
> On Mon, Feb 10, 2020 at 05:05:16PM +0800, Zha Bin wrote:
> > We have compared the number of files and the lines of code between
> > virtio-mmio and virio-pci.
> > 
> > 				Virtio-PCI	    Virtio-MMIO	
> > 	number of files(Linux)	    161			1
> > 	lines of code(Linux)	    78237		538
> 
> 
> 
> Something's very wrong here. virtio PCI is 161 files?
> Are you counting the whole PCI subsystem?

Right, that is just a rough statistics. Surely enough, some drivers will
never get enabled in a typcial config.

> Sure enough:
> 
> $ find drivers/pci -name '*c' |wc -l
> 150

and plus:
$ find arch/x86/pci/ -name '*c' |wc -l
22

> 
> That's not reasonable, this includes a bunch of drivers that
> never run on a typical hypervisor.
> 
> MMIO is also not as small as you are trying to show:
> 
> $ cloc drivers/virtio/virtio_mmio.c include/uapi/linux/virtio_mmio.h
>        2 text files.
>        2 unique files.                              
>        0 files ignored.
> 
> github.com/AlDanial/cloc v 1.82  T=0.01 s (230.7 files/s, 106126.5 lines/s)
> -------------------------------------------------------------------------------
> Language                     files          blank        comment           code
> -------------------------------------------------------------------------------
> C                                1            144            100            535
> C/C++ Header                     1             39             66             36
> -------------------------------------------------------------------------------
> SUM:                             2            183            166            571
> -------------------------------------------------------------------------------
> 
> 
> I don't doubt MMIO is smaller than PCI. Of course that's because it has
> no features to speak of - just this patch already doubles it's size. If
> we keep doing that because we want the features then they will reach
> the same size in about 4 iterations.

Since current virtio-mmio size is small enough, so adding any notable
feature would easily double it. I have no objection that it may one day
reach the same level of PCI, but in this patch some are actually
generic changes and for MSI specific code we provide the option to
confige away.

Thanks,
Chao

> 
> 
> -- 
> MST