Re: [PATCH 4/6] x86/entry: Optimize local_db_save() for virt

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Thu, May 28, 2020 at 10:19:41PM +0200, Peter Zijlstra wrote:
>  static int arch_bp_generic_len(int x86_len)
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3027,6 +3027,8 @@ static int nested_vmx_check_vmentry_hw(s
>  
>  	/*
>  	 * VMExit clears RFLAGS.IF and DR7, even on a consistency check.
> +	 * XXX how is this not broken? access to cpu_dr7 ought to be with
> +	 * IRQs disabled.

Ah, it's simply broken.  This code is conditional on a module param that's
off by default, i.e. it's not run widely, and odds are intersection with
debugging is rare.

Moving local_irq_enable() below the DR7 restoration is not an issue.

Maybe also add lockdep_assert_irqs_disabled() to hw_breakpoint_restore() or
hw_breakpoint_active()?

>  	 */
>  	local_irq_enable();
>  	if (hw_breakpoint_active())
> 
>