linux-kernel - Re: [PATCH] x86/kvm: Don't enable IRQ when IRQ enabled in kvm

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Date:   Mon, 6 Sep 2021 12:32:45 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     Lai Jiangshan <laijs@...ux.alibaba.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org
Subject: Re: [PATCH] x86/kvm: Don't enable IRQ when IRQ enabled in kvm_wait

On 14/08/21 05:51, Lai Jiangshan wrote:
> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> 
> Commit f4e61f0c9add3 ("x86/kvm: Fix broken irq restoration in kvm_wait")
> replaced "local_irq_restore() when IRQ enabled" with "local_irq_enable()
> when IRQ enabled" to suppress a warnning.
> 
> Although there is no similar debugging warnning for doing local_irq_enable()
> when IRQ enabled as doing local_irq_restore() in the same IRQ situation.  But
> doing local_irq_enable() when IRQ enabled is no less broken as doing
> local_irq_restore() and we'd better avoid it.
> 
> Cc: Mark Rutland <mark.rutland@....com>
> Cc: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Lai Jiangshan <laijs@...ux.alibaba.com>
> ---
> 
> The original debugging warnning was introduced in commit 997acaf6b4b5
> ("lockdep: report broken irq restoration").  I think a similar debugging
> check and warnning should also be added to "local_irq_enable() when IRQ
> enabled" and even maybe "local_irq_disable() when IRQ disabled" to detect
> something this:
> 
>      | local_irq_save(flags);
>      | local_irq_disable();
>      | local_irq_restore(flags);
>      | local_irq_enable();
> 
> Or even we can do the check in lockdep+TRACE_IRQFLAGS:
> 
> In lockdep_hardirqs_on_prepare(), lockdep_hardirqs_enabled() was checked
> (and exit) before checking DEBUG_LOCKS_WARN_ON(!irqs_disabled()), so lockdep
> can't give any warning for these kind of situations.  If we did the check
> in lockdep, we would have found the problem before, and we don't need
> 997acaf6b4b5.
> 
> Any thought? Mark? Peter?
> 
>   arch/x86/kernel/kvm.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
> index a26643dc6bd6..b656456c3a94 100644
> --- a/arch/x86/kernel/kvm.c
> +++ b/arch/x86/kernel/kvm.c
> @@ -884,10 +884,11 @@ static void kvm_wait(u8 *ptr, u8 val)
>   	} else {
>   		local_irq_disable();
>   
> +		/* safe_halt() will enable IRQ */
>   		if (READ_ONCE(*ptr) == val)
>   			safe_halt();
> -
> -		local_irq_enable();
> +		else
> +			local_irq_enable();
>   	}
>   }
>   
> 

Queued, thanks.

paolo