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



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

On Fri, Apr 15, 2016 at 10:46 AM, thibaut noah <thibaut noah 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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