Re: [Qemu-devel] [kvmarm] [RFC] Virtio-desktop: Virtio-based virtual desktop

[Gleb Natapov]

On Sun, Jan 27, 2013 at 10:12:20AM +0000, Blue Swirl wrote:
> On Sun, Jan 27, 2013 at 8:20 AM, Gleb Natapov <address@hidden> wrote:
> > On Fri, Jan 25, 2013 at 08:31:54PM +0100, Alexander Graf wrote:
> >>
> >> On 25.01.2013, at 20:04, Blue Swirl wrote:
> >>
> >> > On Thu, Jan 24, 2013 at 6:10 AM, Anup Patel <address@hidden> wrote:
> >> >> Hi All,
> >> >>
> >> >> How about having a generic Virtio-based machine for emulating a virtual
> >> >> desktop ?
> >> >
> >> > I have also thought about this, current virtio design is not very
> >> > clean. On the downside, pure no-legacy approach might not work well if
> >> > you want the host to give control of a host device to guest (VFIO like
> >> > pass through using IOMMUs).
> >> >
> >> >> I know folks have already thought about this and probably also tried
> >> >> something or other on this front but, it will be good to know the 
> >> >> downsides.
> >> >>
> >> >> Virtio-desktop can be a separate specification describing a virtual 
> >> >> desktop.
> >> >> Of-course we cannot avoid few architecture dependent virtual devices in 
> >> >> but
> >> >> the Virtio-desktop specification will try to keep minimum possible
> >> >> architecture dependent devices.
> >> >>
> >> >> As per our thoughts, a Virtio-desktop will have two kinds of devices:
> >> >> 1. Architecture dependent devices: This devices will be emulated 
> >> >> in-kernel
> >> >> by architecture specific code of KVM or Xen or Some other hypervisor.
> >> >>   a) Virtualized CPU
> >> >>   b) Virtualized PIC (i.e. in-kernel architecture specific emulation of
> >> >> irqchip)
> >> >>   c) Virtualized Timer (i.e. in-kernel architecture specific emulation 
> >> >> of
> >> >> timer chip)
> >> >
> >> > I think reusing PIC and timer design is not the most optimal. What a
> >> > PV aware OS really wants is to wake up because of an external event or
> >> > at some specific point of time (or after a specific delay) and easy
> >> > way to manage the VM clock without timer tick counting. With a
> >> > tickless approach, it would need the timer events as rarely as
> >> > possible.
> >> >
> >> > Perhaps a better approach would be to introduce a way for the guest to
> >> > subscribe to a list of external events (including time), which would
> >> > be fed to it via something like reverse hypercall (or just use legacy
> >> > interrupts). Preferably it should be possible to pass any events
> >> > through a stack of guests to the end consumer guest and even
> >> > applications in a guest.
> >>
> >> This approach falls apart once hardware vendors implement timer interrupts 
> >> inside a guest context (which Intel and IIUC ARM are doing). At that 
> >> point, it's actually more efficient to do real timer calls to real 
> >> hardware.
> >>
> > Same with irq chip. Implementing PV irqchip today is going backwards.
> 
> I'm not sure this should be the conclusion. Obviously throwing more
> hardware at virtualization makes it faster and the x86 host will
> probably also benefit from faster interrupt handling as a side
> benefit, so emulating the PIC etc. makes sense for now and also in
> short term (some years from now).
Moder OSes, i.e those that have a non zero chance to get virtio-desktop
support, do not use PIC on x86 after bootup, so there is not performance
gains in replacing it with something PV and I hope you do not propose
that.  You probably think about using PV in place of LAPIC which,
besides have to be written and debugged,  will be slower on modern x86 HW
since mode x86 HW accelerates LAPIC emulation. AFAIK arm also has irq
chip virtualization support in HW.

> 
> But long term, if you could direct the hardware design efforts of
> major companies and standardization bodies to any direction, is fast
> emulation of 30 year old technology what would create the fastest
> architecture in the future? For example, maybe the PIC+LAPIC should be
> replaced in long term by something that can vector the interrupts to
> different VCPUs directly, each at different privilege levels?
>
What prevents you from doing it today with LAPIC/MSIX? PIC/IOAPIC are
legacy and not used for anything performance sensitive.  And do you truly
believe that HW vendors will abandon current HW in favor of your design?

--
                        Gleb.