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Re: [Qemu-devel] [PATCH (resend, rebase) 3/3] virtio-serial: Enable ioev


From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [PATCH (resend, rebase) 3/3] virtio-serial: Enable ioeventfd
Date: Mon, 28 Feb 2011 15:28:49 +0000

On Mon, Feb 28, 2011 at 11:12 AM, Amit Shah <address@hidden> wrote:
> Enable ioeventfd for virtio-serial devices by default.  Commit
> 25db9ebe15125deb32958c6df74996f745edf1f9 lists the benefits of using
> ioeventfd.
>
> Copying a file from guest to host over a virtio-serial channel didn't
> show much difference in time or io_exit rate.

The cost of enabling ioeventfd is one eventfd file descriptor and KVM
in-kernel device slot per virtqueue.  The current maximum number per
VM is 200, this is a kernel limit in
include/linux/kvm_host.h:NR_IOBUS_DEVS.

Do you really want to use ioeventfd for virtio-serial?  Perhaps this
is more useful for high-frequency device interfaces.

Stefan



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