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

[Paolo Bonzini]

On 23/07/19 11:47, Stefan Hajnoczi wrote:
> fw_cfg could expose this information so guest kernels know when to
> stop enumerating the PCI bus.  This would make all PCI guests with new
> kernels boot ~50 ms faster, regardless of machine type.

The number of buses is determined by the firmware, not by QEMU, so
fw_cfg would not be the right interface.  In fact (as I have just
learnt) lastbus is an x86-specific option that overrides the last bus
returned by SeaBIOS's handle_1ab101.

So the next step could be to figure out what is the lastbus returned by
handle_1ab101 and possibly why it isn't zero.

Paolo

> The difference between microvm and tuned Q35 is 76 ms now.
> 
> microvm:
> qemu_init_end: 64.043264
> linux_start_kernel: 65.481782 (+1.438518)
> linux_start_user: 114.938353 (+49.456571)
> 
> Q35 with -cpu host and pci=lasbus=0:
> qemu_init_end: 73.711569
> linux_start_kernel: 113.414311 (+39.702742)
> linux_start_user: 190.949939 (+77.535628)
> 
> There is a ~39 ms difference before linux_start_kernel.  SeaBIOS is
> loading the PVH Option ROM.
> 
> Stefano: any recommendations for profiling or tuning SeaBIOS?