Re: [PATCH v2 4/9] microvm: add usb support

[Gerd Hoffmann]

On Tue, Oct 06, 2020 at 12:03:26PM +0200, Igor Mammedov wrote:
> On Thu,  1 Oct 2020 10:28:29 +0200
> Gerd Hoffmann <kraxel@redhat.com> wrote:
> 
> > Wire up "usb=on" machine option, when enabled add
> > a sysbus xhci controller with 8 ports.
> 
> first pci, and now usb - not so micro anymore.

Both pcie and usb are optional and disabled by default.

> why not just reuse pci to add usb controller?
> what/how much do we win avoiding PCI?

Didn't benchmark stuff myself, but there are several reports that
the pcie initialization by the linux kernel at boot is noticable
(lots of vmexits for pci config space access).

IIRC this is also the reason why microvm started without pci support in
the first place.  So being able to enable usb without having to enable
pcie too looks useful to me.

But, yes, functionality-wise there isn't much of a difference between
"-microvm usb=on" and "-microvm pcie=on -device qemu-xhci".

take care,
  Gerd