linux-kernel - Re: [PATCH v3 15/17] arm64: Add CNT{P,V}CTSS_EL0 alternatives to cnt{p,v}ct

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Date:   Mon, 11 Oct 2021 11:33:09 +0100
From:   Will Deacon <will@...nel.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Mark Rutland <mark.rutland@....com>,
        Daniel Lezcano <daniel.lezcano@...aro.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Shier <pshier@...gle.com>,
        Raghavendra Rao Ananta <rananta@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Linus Walleij <linus.walleij@...aro.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH v3 15/17] arm64: Add CNT{P,V}CTSS_EL0 alternatives to
 cnt{p,v}ct_el0

On Sun, Oct 10, 2021 at 12:43:04PM +0100, Marc Zyngier wrote:
> CNTPCTSS_EL0 and CNTVCTSS_EL0 are alternatives to the usual
> CNTPCT_EL0 and CNTVCT_EL0 that do not require a previous ISB
> to be synchronised (SS stands for Self-Synchronising).
> 
> Use the ARM64_HAS_ECV capability to control alternative sequences
> that switch to these low(er)-cost primitives. Note that the
> counter access in the VDSO is for now left alone until we decide
> whether we want to allow this.
> 
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> ---
>  arch/arm64/include/asm/arch_timer.h | 32 +++++++++++++++++++++--------
>  arch/arm64/include/asm/sysreg.h     |  3 +++
>  2 files changed, 27 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/arch_timer.h b/arch/arm64/include/asm/arch_timer.h
> index 519ac1f7f859..33a08fff0f06 100644
> --- a/arch/arm64/include/asm/arch_timer.h
> +++ b/arch/arm64/include/asm/arch_timer.h
> @@ -64,14 +64,26 @@ DECLARE_PER_CPU(const struct arch_timer_erratum_workaround *,
>  
>  static inline notrace u64 arch_timer_read_cntpct_el0(void)
>  {
> -	isb();
> -	return read_sysreg(cntpct_el0);
> +	u64 cnt;
> +
> +	asm volatile(ALTERNATIVE("isb\n mrs %x0, cntpct_el0",
> +				 "nop\n" __mrs_s("%x0", SYS_CNTPCTSS_EL0),
> +				 ARM64_HAS_ECV)
> +		     : "=r" (cnt));
> +
> +	return cnt;

Why do you need to use %x0 instead of just %0 here? Similarly for the other
functions you're changing in this file.

Will