Re: [PATCH v2] x86/shstk: Provide kernel command line knob to disable

[Edgecombe, Rick P]

On Wed, 2026-05-13 at 22:51 +0200, Mathias Krause wrote:
> On 08.05.26 18:35, Edgecombe, Rick P wrote:
> > On Fri, 2026-05-08 at 09:23 +0200, Mathias Krause wrote:
> > > > Now that KVM uses this this feature independently of X86_FEATURE_USER_SHSTK,
> > > > it might be good to have the plain HW shstk feature exposed for just normal
> > > > runtime user use. (+Chao, for KVM CET)
> > > 
> > > But that sounds more like having the need for an official chicken bit,
> > > like I was proposing, no? Using 'clearcpuid=shstk' as a workaround for
> > > whatever KVM bugs, similar in spirit to 'nousershstk', but without the
> > > kernel taint?
> > 
> > For users to turn off shadow stack for guests? You can do this via the KVM API
> > in the normal way you customize guests.
> 
> https://git.kernel.org/linus/2d5d3fc593c9b7e41bee86175d7b9e11f470072e
> 
> Oh, well....

Heh, well, the clearcpuid would have helped debugging I guess. And that module
param turns off shadow stack for KVM, but not for userspace. So doesn't help
your CR4.CET problems I guess.

Are you planning to send another revision of the clearcpuid approach? I'm
convinced something like this is a good thing to have, so I'll probably pick it
up if you don't.