linux-kernel - Re: [PATCH 1/6] x86/entry: Introduce local_db

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Date:   Thu, 28 May 2020 21:52:30 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Peter Zijlstra <peterz@...radead.org>, tglx@...utronix.de,
        luto@...capital.net
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        sean.j.christopherson@...el.com, daniel.thompson@...aro.org
Subject: Re: [PATCH 1/6] x86/entry: Introduce local_db_{save,restore}()

On 28/05/2020 21:19, Peter Zijlstra wrote:
> --- a/arch/x86/include/asm/debugreg.h
> +++ b/arch/x86/include/asm/debugreg.h
> @@ -113,6 +113,31 @@ static inline void debug_stack_usage_inc
>  static inline void debug_stack_usage_dec(void) { }
>  #endif /* X86_64 */
>  
> +static __always_inline void local_db_save(unsigned long *dr7)
> +{
> +	get_debugreg(*dr7, 7);
> +	if (*dr7)
> +		set_debugreg(0, 7);

%dr7 has an architecturally stuck bit in it.

You want *dr7 != 0x400 to avoid writing 0 unconditionally.

Also, API wise, wouldn't it be nicer to write "dr7 = local_db_save()"
rather than having a void function returning a single long via pointer?

~Andrew