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

[Sean Christopherson <seanjc@google.com>]

On Tue, Oct 05, 2021, Kuppuswamy Sathyanarayanan wrote:
> From: Xiaoyao Li <xiaoyao.li@intel.com>
> 
> 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@intel.com>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> ---
>  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
>