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    SubjectRe: [PATCHv5 3/3] vhost_net: a kernel-level virtio server
    On Wed, Sep 16, 2009 at 05:22:37PM +0200, Arnd Bergmann wrote:
    > On Wednesday 16 September 2009, Michael S. Tsirkin wrote:
    > > On Wed, Sep 16, 2009 at 04:57:42PM +0200, Arnd Bergmann wrote:
    > > > On Tuesday 15 September 2009, Michael S. Tsirkin wrote:
    > > > > Userspace in x86 maps a PCI region, uses it for communication with ppc?
    > > >
    > > > This might have portability issues. On x86 it should work, but if the
    > > > host is powerpc or similar, you cannot reliably access PCI I/O memory
    > > > through copy_tofrom_user but have to use memcpy_toio/fromio or readl/writel
    > > > calls, which don't work on user pointers.
    > > >
    > > > Specifically on powerpc, copy_from_user cannot access unaligned buffers
    > > > if they are on an I/O mapping.
    > > >
    > > We are talking about doing this in userspace, not in kernel.
    >
    > Ok, that's fine then. I thought the idea was to use the vhost_net driver

    It's a separate issue. We were talking generally about configuration
    and setup. Gregory implemented it in kernel, Avi wants it
    moved to userspace, with only fastpath in kernel.

    > to access the user memory, which would be a really cute hack otherwise,
    > as you'd only need to provide the eventfds from a hardware specific
    > driver and could use the regular virtio_net on the other side.
    >
    > Arnd <><

    To do that, maybe copy to user on ppc can be fixed, or wrapped
    around in a arch specific macro, so that everyone else
    does not have to go through abstraction layers.

    --
    MST


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