Re: [PATCH 1/2] x86/split_lock: Don't try to handle user split lock in TDX guest

[Xiaoyao Li]

On 11/26/2025 7:25 PM, Kiryl Shutsemau wrote:
> On Wed, Nov 26, 2025 at 06:02:03PM +0800, Xiaoyao Li wrote:
> > When the host enables split lock detection feature, the split lock from
> > guests (normal or TDX) triggers #AC. The #AC caused by split lock access
> > within a normal guest triggers a VM Exit and is handled in the host.
> > The #AC caused by split lock access within a TDX guest does not trigger
> > a VM Exit and instead it's delivered to the guest self.
> >
> > The default "warning" mode of handling split lock depends on being able
> > to temporarily disable detection to recover from the split lock event.
> > But the MSR that disables detection is not accessible to a guest.
> >
> > This means that TDX guests today can not disable the feature or use
> > the "warning" mode (which is the default). But, they can use the "fatal"
> > mode.
> >
> > Force TDX guests to use the "fatal" mode.
> >
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > ---
> >   arch/x86/kernel/cpu/bus_lock.c | 17 ++++++++++++++++-
> >   1 file changed, 16 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/cpu/bus_lock.c b/arch/x86/kernel/cpu/bus_lock.c
> > index 981f8b1f0792..f278e4ea3dd4 100644
> > --- a/arch/x86/kernel/cpu/bus_lock.c
> > +++ b/arch/x86/kernel/cpu/bus_lock.c
> > @@ -315,9 +315,24 @@ void bus_lock_init(void)
> >   	wrmsrq(MSR_IA32_DEBUGCTLMSR, val);
> >   }
> >   
> > +static bool split_lock_fatal(void)
> > +{
> > +	if (sld_state == sld_fatal)
> > +		return true;
> > +
> > +	/*
> > +	 * TDX guests can not disable split lock detection.
> > +	 * Force them into the fatal behavior.
> > +	 */
> > +	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
> > +		return true;
> > +
> > +	return false;
> > +}
> > +
> >   bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> >   {
> > -	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> > +	if ((regs->flags & X86_EFLAGS_AC) || split_lock_fatal())
> >   		return false;
>
> Maybe it would be cleaner to make it conditional on
> cpu_model_supports_sld instead of special-casing TDX guest?
>
> #AC on any platfrom when we didn't asked for it suppose to be fatal, no?

But TDX is the only one has such special non-architectural behavior.

For example, for normal VMs under KVM, the behavior is x86 
architectural. MSR_TEST_CTRL is not accessible to normal VMs, and no 
split lock #AC will be delivered to the normal VMs because it's handled 
by KVM.

> >   	split_lock_warn(regs->ip);
> >   	return true;
> > --
> > 2.43.0