Re: [Qemu-devel] [RFC] Next gen kvm api

From: Anthony Liguori
Date: Mon Feb 06 2012 - 14:11:59 EST

On 02/06/2012 11:41 AM, Rob Earhart wrote:
On Sun, Feb 5, 2012 at 5:14 AM, Avi Kivity<avi@xxxxxxxxxx> wrote:
On 02/03/2012 12:13 AM, Rob Earhart wrote:
On Thu, Feb 2, 2012 at 8:09 AM, Avi Kivity<avi@xxxxxxxxxx
<mailto:avi@xxxxxxxxxx>> wrote:

The kvm api has been accumulating cruft for several years now.
This is
due to feature creep, fixing mistakes, experience gained by the
maintainers and developers on how to do things, ports to new
architectures, and simply as a side effect of a code base that is
developed slowly and incrementally.

While I don't think we can justify a complete revamp of the API
now, I'm
writing this as a thought experiment to see where a from-scratch
API can
take us. Of course, if we do implement this, the new and old APIs
have to be supported side by side for several years.

kvm currently uses the much-loved ioctl() system call as its entry
point. While this made it easy to add kvm to the kernel
it does have downsides:

- overhead in the entry path, for the ioctl dispatch path and vcpu
(low but measurable)
- semantic mismatch: kvm really wants a vcpu to be tied to a
thread, and
a vm to be tied to an mm_struct, but the current API ties them to file
descriptors, which can move between threads and processes. We check
that they don't, but we don't want to.

Moving to syscalls avoids these problems, but introduces new ones:

- adding new syscalls is generally frowned upon, and kvm will need
- syscalls into modules are harder and rarer than into core kernel
- will need to add a vcpu pointer to task_struct, and a kvm pointer to

Syscalls that operate on the entire guest will pick it up implicitly
from the mm_struct, and syscalls that operate on a vcpu will pick
it up
from current.


I like the ioctl() interface. If the overhead matters in your hot path,

I can't say that it's a pressing problem, but it's not negligible.

I suspect you're doing it wrong;

What am I doing wrong?

"You the vmm" not "you the KVM maintainer" :-)

To be a little more precise: If a VCPU thread is going all the way out
to host usermode in its hot path, that's probably a performance
problem regardless of how fast you make the transitions between host
user and host kernel.

That's why ioctl() doesn't bother me. I think it'd be more useful to
focus on mechanisms which don't require the VCPU thread to exit at all
in its hot paths, so the overhead of the ioctl() really becomes lost
in the noise. irq fds and ioevent fds are great for that, and I
really like your MMIO-over-socketpair idea.

I'm not so sure. ioeventfds and a future mmio-over-socketpair have to put the kthread to sleep while it waits for the other end to process it. This is effectively equivalent to a heavy weight exit. The difference in cost is dropping to userspace which is really neglible these days (< 100 cycles).

There is some fast-path trickery to avoid heavy weight exits but this presents the same basic problem of having to put all the device model stuff in the kernel.

ioeventfd to userspace is almost certainly worse for performance. And Avi mentioned, you can emulate this behavior yourself in userspace if so inclined.


Anthony Liguori
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at