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

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

On Tue, Oct 21, 2025 at 03:35:28PM +0000, Jon Kohler wrote:
> 
> 
> > On Oct 21, 2025, at 11:27 AM, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> > 
> > > > 
> > 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://urldefense.proofpoint.com/v2/url?u=https-3A__blogs.oracle.com_linux_post_qemu-2Dlive-2Dupdate&d=DwIFaQ&c=s883GpUCOChKOHiocYtGcg&r=NGPRGGo37mQiSXgHKm5rCQ&m=UUSvpPViiTB6CJQtj3GREK4bQFz7MT9uNiAu5AL3O23d6I1yk4vheLDyR41ZcbRI&s=x19gwHU3HGXSlGkK0ppkawex3SIbs8xHj5hPtwNCFwc&e= 
> > 
> > That should solve your QEMU problem.
> 
> Can this live update feature change CPU feature bits (e.g. add on
> -cpu ModelHere … new_flag=yes) during the update?

It is possible, but a pain in operation b/c you have to keep track of
the newer parameters you are adding and if add some device in wrong
order (wrong PCI BDF), well, things won't be nice.
> 
> Can it change CPU models during the live update (e.g. change
> -cpu Model-v1 to -cpu Model-v2)?

Not exactly. But you can modify the built-in Model-v1 (in the newer
version) to have the newer CPU flags and it will carry-over. 

Anyhow it is all in the latest version of QEMU.