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

[Xiaoyao Li]

On 10/14/2021 5:32 AM, Sathyanarayanan Kuppuswamy wrote:
> + 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.

What if hypervisor doesn't intercept #AC and host enables 
SPLIT_LOCK_DETECT during guest running? That's exactly the case TDX is 
facing.

(BTW, this patch is not complete that no matter what state sld_state is, 
it should always treat it as fatal even sld_warn because sld_warn 
doesn't guarantee SPLIT_LOCK_DETECT is available)

Sathya, we need to drop this one. Andi has anther one to disable split 
lock detection for SPR. That's a better direction.

> > > 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