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    SubjectRe: [PULL] virtio
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    On Tue, 2011-11-01 at 14:42 +0200, Michael S. Tsirkin wrote:
    > On Tue, Nov 01, 2011 at 02:33:33PM +0200, Sasha Levin wrote:
    > > On Tue, 2011-11-01 at 13:45 +0200, Michael S. Tsirkin wrote:
    > > > On Tue, Nov 01, 2011 at 10:39:08AM +1030, Rusty Russell wrote:
    > > > > * [new tag] rusty@rustcorp.com.au-v3.1-7196-gac5be1e -> rusty@rustcorp.com.au-v3.1-7196-gac5be1e
    > > > >
    > > > > The following changes since commit 839d8810747bbf39e0a5a7f223b67bffa7945f8d:
    > > > >
    > > > > Merge branch 'i2c-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/jdelvare/staging (2011-10-30 15:54:59 -0700)
    > > > >
    > > > > are available in the git repository at:
    > > > >
    > > > > git://github.com/rustyrussell/linux.git master
    > > > >
    > > > > Alexey Kardashevskiy (1):
    > > > > virtio-pci: Use PCI MMIO instead of PIO when available
    > > >
    > > > I missed this one - wasn't Cc'd neither me, kvm or virtio mailing lists.
    > > >
    > > > It's well known that mmio is much slower than pio on kvm, since
    > > > mmio needs to be emulated to get at the address.
    > > > So I'm expecting this will cause a performance regression.
    > > > IMO we should keep using PIO for VQ and interrupt status access
    > > > if PIO is available.
    > > >
    > > > Another consideration is that in an attempt to pack data
    > > > densely in the PIO space the layout became messy.
    > > > It would be better to have common config space and
    > > > per-device config space in separate pages, possibly
    > > > with padding between them.
    > > >
    > > > So I'd like a bit more discussion on this patch,
    > > > I'm concerned that if this is released in 3.2 as is we'll
    > > > have to support this forever. How about a revert for now?
    > >
    > > Another thing, the patch tries to map BAR 2 and use it as the
    > > configuration space.
    > >
    > > It's both not documented properly anywhere, and is not fully backwards
    > > compatible - we currently use BAR 2 as part of our MSIX handling in the
    > > kvm tool and I'm sure we're not the only ones to assume virtio-pci only
    > > uses BAR 0.
    > >
    > > A proper solution would be for example a configuration in the PIO config
    > > space which points to the MMIO BAR to use instead.
    >
    > I think it makes sense to put the configuration in PCI
    > configuration space, using vendor-specific capability.
    > This way we can reuse existing functionality for scanning
    > capability lists.

    Yup, I agree. It would also allow dropping the PIO config BAR altogether
    after some period of backwards compatibility.

    It was mostly to point out that the patch isn't really backwards
    compatible :)

    --

    Sasha.



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