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

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/05/2012 03:51 AM, Gleb Natapov wrote:
> On Sun, Feb 05, 2012 at 11:44:43AM +0200, Avi Kivity wrote:
>> On 02/05/2012 11:37 AM, Gleb Natapov wrote:
>>> On Thu, Feb 02, 2012 at 06:09:54PM +0200, Avi Kivity wrote:
>>>> Device model
>>>> ------------
>>>> Currently kvm virtualizes or emulates a set of x86 cores, with or
>>>> without local APICs, a 24-input IOAPIC, a PIC, a PIT, and a number of
>>>> PCI devices assigned from the host.  The API allows emulating the local
>>>> APICs in userspace.
>>>>
>>>> The new API will do away with the IOAPIC/PIC/PIT emulation and defer
>>>> them to userspace.  Note: this may cause a regression for older guests
>>>> that don't support MSI or kvmclock.  Device assignment will be done
>>>> using VFIO, that is, without direct kvm involvement.
>>>>
>>> So are we officially saying that KVM is only for modern guest
>>> virtualization?
>>
>> No, but older guests may have reduced performance in some workloads
>> (e.g. RHEL4 gettimeofday() intensive workloads).
>>
> Reduced performance is what I mean. Obviously old guests will continue working.

An interesting solution to this problem would be an in-kernel device VM.

Most of the time, the hot register is just one register within a more complex 
device.  The reads are often side-effect free and trivially computed from some 
device state + host time.

If userspace had a way to upload bytecode to the kernel that was executed for a 
PIO operation, it could either pass the operation to userspace or handle it 
within the kernel when possible without taking a heavy weight exit.

If the bytecode can access variables in a shared memory area, it could be pretty 
efficient to work with.

This means that the kernel never has to deal with specific in-kernel devices but 
that userspace can accelerator as many of its devices as it sees fit.

This could replace ioeventfd as a mechanism (which would allow clearing the 
notify flag before writing to an eventfd).

We could potentially just use BPF for this.

Regards,

Anthony Liguori