linux-kernel - Re: [patch V4 part 1 27/36] arm64: Prepare arch_nmi

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Message-ID: <427895535.20271.1589412514423.JavaMail.zimbra@efficios.com>
Date:   Wed, 13 May 2020 19:28:34 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>,
        paulmck <paulmck@...nel.org>, Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Frederic Weisbecker <frederic@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Petr Mladek <pmladek@...e.com>, rostedt <rostedt@...dmis.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>,
        Brian Gerst <brgerst@...il.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>
Subject: Re: [patch V4 part 1 27/36] arm64: Prepare arch_nmi_enter() for
 recursion

----- On May 5, 2020, at 9:16 AM, Thomas Gleixner tglx@...utronix.de wrote:

> +#define arch_nmi_enter()						\
[...]							\
> +	___hcr = read_sysreg(hcr_el2);					\
> +	if (!(___hcr & HCR_TGE)) {					\
> +		write_sysreg(___hcr | HCR_TGE, hcr_el2);		\
> +		isb();							\

Why is there an isb() above ^ ....

> +	}								\
> +	/*								\
[...]
> -#define arch_nmi_exit()								\
[...]
> +	/*								\
> +	 * Make sure ___ctx->cnt release is visible before we		\
> +	 * restore the sysreg. Otherwise a new NMI occurring		\
> +	 * right after write_sysreg() can be fooled and think		\
> +	 * we secured things for it.					\
> +	 */								\
> +	barrier();							\
> +	if (!___ctx->cnt && !(___hcr & HCR_TGE))			\
> +		write_sysreg(___hcr, hcr_el2);				\

And not here ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com