Re: [Qemu-devel] [PATCH v3 0/4] Introduce the microvm machine type

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jul 25, 2019 at 01:01:29PM +0100, Stefan Hajnoczi wrote:
> On Thu, Jul 25, 2019 at 12:23 PM Paolo Bonzini <pbonzini@redhat.com> wrote:
> > On 25/07/19 12:42, Sergio Lopez wrote:
> > > Peter Maydell <peter.maydell@linaro.org> writes:
> > >> On Thu, 25 Jul 2019 at 10:59, Michael S. Tsirkin <mst@redhat.com> wrote:
> > >>> OK so please start with adding virtio 1 support. Guest bits
> > >>> have been ready for years now.
> > >>
> > >> I'd still rather we just used pci virtio. If pci isn't
> > >> fast enough at startup, do something to make it faster...
> > >
> > > Actually, removing PCI (and ACPI), is one of the main ways microvm has
> > > to reduce not only boot time, but also the exposed surface and the
> > > general footprint.
> > >
> > > I think we need to discuss and settle whether using virtio-mmio (even if
> > > maintained and upgraded to virtio 1) for a new machine type is
> > > acceptable or not. Because if it isn't, we should probably just ditch
> > > the whole microvm idea and move to something else.
> >
> > I agree.  IMNSHO the reduced attack surface from removing PCI is
> > (mostly) security theater, however the boot time numbers that Sergio
> > showed for microvm are quite extreme and I don't think there is any hope
> > of getting even close with a PCI-based virtual machine.
> >
> > So I'd even go a step further: if using virtio-mmio for a new machine
> > type is not acceptable, we should admit that boot time optimization in
> > QEMU is basically as good as it can get---low-hanging fruit has been
> > picked with PVH and mmap is the logical next step, but all that's left
> > is optimizing the guest or something else.
> 
> I haven't seen enough analysis to declare boot time optimization done.
> QEMU startup can be profiled and improved.

Right, and that will always stay the case. OTOH imho microvm is
non-intrusive enough, and small enough, that we'd just put it upstream
after addressing low-level comments.
This will allow more contributions from people interested in boot time.
With no cross-version migration support, or maybe migration
disabled completely, maintainance burden should not be too high.
Not everyone wants to hack on pci/acpi specifically.


> The numbers show that removing PCI and ACPI makes things faster but
> this doesn't justify removing them.  Understanding of why they are
> slow is what justifies removing them.  Otherwise it could just be a
> misconfiguration, inefficient implementation, etc and we've seen there
> is low-hanging fruit.
> 
> How much time is spent doing PCI initialization?  Is the vmexit
> pattern for PCI initialization as good as the hardware interface
> allows?

I know in the bios we wanted to use memory mapped for pci config
accesses for a very long time now. This makes each vmexit slower but
cuts the number of exits by half. Only affects seabios though.




> Without an analysis of why things are slow it's not possible come to
> an informed decision.
> 
> Stefan