Re: [PATCH v7 6/6] x86/split_lock: Fix the split lock #AC handling when running as guest

[Sathyanarayanan Kuppuswamy]

+ Xiaoyao

On 10/13/21 1:30 PM, Sean Christopherson wrote:
> On Tue, Oct 05, 2021, Kuppuswamy Sathyanarayanan wrote:
> > From: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> >
> > If running as guest and hypervisor enables
> > MSR_TEST_CTRL.SPLIT_LOCK_DETECT during its running, it can get split
> > lock #AC even though sld_state is sld_off.
> That's a hypervisor bug, no?  The hypervisor should never inject a fault that
> the guest cannot reasonably expect.
>
> > For kernel mode #AC, it always dies("split lock"), no more action
> > needed.
> >
> > For user mode #AC, it should treat sld_off (default state when feature
> > is not available) as fatal as well.
> >
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
> > ---
> >   arch/x86/kernel/cpu/intel.c | 7 ++++++-
> >   1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
> > index 01d7935feaed..47f0bc95ce2a 100644
> > --- a/arch/x86/kernel/cpu/intel.c
> > +++ b/arch/x86/kernel/cpu/intel.c
> > @@ -1190,7 +1190,12 @@ static void bus_lock_init(void)
> >   
> >   bool handle_user_split_lock(struct pt_regs *regs, long error_code)
> >   {
> > -	if ((regs->flags & X86_EFLAGS_AC) || sld_state == sld_fatal)
> > +	/*
> > +	 * In virtualization environment, it can get split lock #AC even when
> > +	 * sld_off but hypervisor enables it.
> > +	 * Thus only handles when sld_warn explicitly.
> > +	 */
> > +	if ((regs->flags & X86_EFLAGS_AC) || sld_state != sld_warn)
> >   		return false;
> >   	split_lock_warn(regs->ip);
> >   	return true;
> > --
> > 2.25.1
--
Sathyanarayanan Kuppuswamy
Linux Kernel Developer