RE: [PATCH 1/1] x86/hyperv: Disable IBT when hypercall page lacks ENDBR instruction

[Michael Kelley (LINUX)]

From: Michael Kelley (LINUX) <mikelley@xxxxxxxxxxxxx> Sent: Thursday, July 20, 2023 5:42 PM
> 
> From: Peter Zijlstra <peterz@xxxxxxxxxxxxx> Sent: Thursday, July 20, 2023 2:16 PM
> >
> > > @@ -472,6 +473,26 @@ void __init hyperv_init(void)
> > >  	}
> > >
> > >  	/*
> > > +	 * Some versions of Hyper-V that provide IBT in guest VMs have a bug
> > > +	 * in that there's no ENDBR64 instruction at the entry to the
> > > +	 * hypercall page. Because hypercalls are invoked via an indirect call
> > > +	 * to the hypercall page, all hypercall attempts fail when IBT is
> > > +	 * enabled, and Linux panics. For such buggy versions, disable IBT.
> > > +	 *
> > > +	 * Fixed versions of Hyper-V always provide ENDBR64 on the hypercall
> > > +	 * page, so if future Linux kernel versions enable IBT for 32-bit
> > > +	 * builds, additional hypercall page hackery will be required here
> > > +	 * to provide an ENDBR32.
> > > +	 */
> > > +#ifdef CONFIG_X86_KERNEL_IBT
> > > +	if (cpu_feature_enabled(X86_FEATURE_IBT) &&
> > > +	    *(u32 *)hv_hypercall_pg != gen_endbr()) {
> > > +		setup_clear_cpu_cap(X86_FEATURE_IBT);
> > > +		pr_info("Hyper-V: Disabling IBT because of Hyper-V bug\n");
> > > +	}
> > > +#endif
> >
> > pr_warn() perhaps?
> 
> I wanted pr_info() so there's an immediate way to check for this
> case in the dmesg output if a user complains about IBT not being
> enabled when he expects it.   In some sense, the message is temporary
> because once the Hyper-V patch is available and users install it,
> the message will go away.  The pipeline for the Hyper-V patch is a
> bit long, so availability is at least several months away.  This Linux
> workaround will be available much faster.  Once it is picked up on
> stable branches, we will avoid the situations like we saw where
> someone upgraded Fedora 38 from a 6.2 to a 6.3 kernel, and the 6.3
> kernel wouldn't boot because it has kernel IBT enabled.
> 

I realized in the middle of the night that my reply was nonsense. :-(
pr_warn() makes the message visible when pr_info() might not.  I'm
happy to change to pr_warn().

Michael