linux-kernel - Re: [PATCH 4/6] x86/entry: Optimize local_db

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Date:   Fri, 29 May 2020 10:24:49 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, luto@...capital.net,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        andrew.cooper3@...rix.com, daniel.thompson@...aro.org,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 4/6] x86/entry: Optimize local_db_save() for virt

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())
> 
>