[vfio-users] are there performance benefits when use UEFI? instead Legacy BIOS

[Alex Williamson]

On Tue, 9 Feb 2016 12:48:42 +0100
<rudeboyjamaica at gmail.com> wrote:

> Hi all i want to ask if UEFI/OVMF based VM provide some benefits as Legacy
> BIOS based VM is there less IRQ, more performance or something else?
> 
> I still use legacy bios based machine.

The VGA space itself is a shared resource, so every time the guest
tries to access VGA space it gets gets trapped into QEMU, which
forwards the request to VFIO, which negotiates with the VGA arbiter and
adjusts chipset routing as necessary.  Therefore VGA mode is bearable
when there's one consumer, once you start getting contention and the
arbiter needs to switch routing on each access, this can certainly
become a bottleneck.  However, even when using a legacy BIOS with
x-vga, those VGA accesses *only* occur during early boot or if you're
using non-accelerated drivers, so the window for contention is very
small.  Once the guest is up and running, access to legacy VGA space
ceases, and there's no performance difference between legacy BIOS and
UEFI that I'm aware of.  FWIW, it's also a misconception that Q35 based
VMs provide some sort of performance advantage vs 440FX.  The chipset
is virtual, it has no bearing on performance.  Thanks,

Alex