[vfio-users] Looking for performance tips, qemu machine type and pci/pcie-root questionning?

Alex Williamson alex.l.williamson at gmail.com
Fri Apr 15 04:30:22 UTC 2016

On Thu, Apr 14, 2016 at 10:13 PM, Okky Hendriansyah <okky.htf at gmail.com>

> Hi Thibaut,
> On Fri, Apr 15, 2016 at 10:46 AM, thibaut noah <thibaut.noah at gmail.com>
> wrote:
>> Hello everyone, so i'm still in my endless pursuit of performance.
>> Currently crawling qemu options a bit, running the command "$
>> qemu-system-x86_64 -machine help" seems that the main machine for us are
>> q35 and pci-i440FX.
>> From what i read in my online research the main advantage of q35 seems to
>> be the support of pcie controller instead of just pci, so it might give
>> more performance?
> I think Alex had mentioned about this, and if I recall correctly using
> pc-i440fx is preferrable since it is simpler and going to pc-q35 won't have
> any performance benefit. Currently I only use pc-q35 specifically just for
> my Hackintosh guest. I never done any benchmark between these two types
> recently though, so the result might change.

Correct, this seems to be a common misconception.  Neither the VM nor the
assigned devices are going to inherently run faster on a Q35 or a 440FX
machine time.  This is like painting your car red to improve the
performance.  The physical PCIe link is running at full speed even though
the device appears in the guest as a conventional PCI device.  Unless
there's some feature of the card that can be enabled by the VM in PCIe
extended config space, the VM chipset doesn't matter.  It's possible there
are or will be such features, but I don't know of any turbo switches there
now.  Thanks.
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