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SubjectRe: [PATCH v2 0/5] virtio mmio specification enhancement
On Tue, Feb 11, 2020 at 04:05:41PM +0000, Chao Peng wrote:
> 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.

Please try not to make them look so wrong then.
E.g. you don't include drivers/base/platform-msi.c for
mmio do you? Your patch brings a bunch of code in there.

> 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

But what's the point? This is code that is maintained by PCI core
people anyway.

> >
> > 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.

But really unlike PCI this is just PV stuff that is not reused by
anyone. We end up maintaining all this by ourselves.

> 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

The option will make it fall down at runtime but
it does not actually seem to remove all of the overhead.



> >
> >
> > --
> > MST

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