Re: [PATCH] x86/its: use Sapphire Rapids+ feature to opt out

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Oct 21, 2025 at 02:39:15PM +0000, Jon Kohler wrote:
> 
> 
> > On Oct 21, 2025, at 10:01 AM, Dave Hansen <dave.hansen@intel.com> wrote:
> > 
> > > > 
> > On 10/21/25 06:40, Jon Kohler wrote:
> >> So to simplify it down:
> >> A guest VM that updates to a ITS-enabled guest kernel sees performance
> >> impacts on non-vulnerable hardware, when running on non-BHI_CTRL and/or
> >> non-ITS_NO hypervisors, which is a very easy situation to get into, especially
> >> on QEMU with live migration-enabled pools.
> > 
> > By non-$FEATURE, do you mean that they chose to not enumerate those
> > features, or that they are completely ignorant of them?
> 
> Both cases are true for QEMU.
> 
> For ITS_NO, it is an allowed feature, but its not part of a QEMU model by
> default, so the higher level control plane whatever that may be would need to
> specifically turn it on, its not automatic.
> 
> For BHI_CTRL, depending on what QEMU the VM was originally *started* on,
> the guest may have access to Sapphire Rapids models, but BHI_CTRL may
> not have existed in the QEMU source at that time, as those were introduced
> into two different timeframes.

QEMU provides now a mechanism to update itself to a newer version. See

https://blogs.oracle.com/linux/post/qemu-live-update

That should solve your QEMU problem.