Re: [Xen-devel] [PATCH 05/17] xen/arm64: head: Introduce print_reg

From: Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>
To: Julien Grall <julien.grall@arm.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Oleksandr Tyshchenko <Oleksandr_Tyshchenko@epam.com>,
	Stefano Stabellini <sstabellini@kernel.org>,
	Andrii Anisov <Andrii_Anisov@epam.com>,
	"andre.przywara@arm.com" <andre.przywara@arm.com>
Subject: Re: [Xen-devel] [PATCH 05/17] xen/arm64: head: Introduce print_reg
Date: Mon, 15 Jul 2019 18:46:57 +0000	[thread overview]
Message-ID: <87v9w3rv8f.fsf@epam.com> (raw)
In-Reply-To: <20190610193215.23704-6-julien.grall@arm.com>

Hi Julien,

Julien Grall writes:

> At the moment, the user should save x30/lr if it cares about it.
>
> Follow-up patches will introduce more use of putn in place where lr
> should be preserved.
>
> Furthermore, any user of putn should also move the value to register x0
> if it was stored in a different register.
>
> For convenience, a new macro is introduced to print a given register.
> The macro will take care for us to move the value to x0 and also
> preserve lr.
>
> Lastly the new macro is used to replace all the callsite of putn. This
> will simplify rework/review later on.
>
> Note that CurrentEL is now stored in x5 instead of x4 because the latter
> will be clobbered by the macro print_reg.
>
> Signed-off-by: Julien Grall <julien.grall@arm.com>
> ---
>  xen/arch/arm/arm64/head.S | 29 ++++++++++++++++++++++-------
>  1 file changed, 22 insertions(+), 7 deletions(-)
>
> diff --git a/xen/arch/arm/arm64/head.S b/xen/arch/arm/arm64/head.S
> index 84e26582c4..9142b4a774 100644
> --- a/xen/arch/arm/arm64/head.S
> +++ b/xen/arch/arm/arm64/head.S
> @@ -90,8 +90,25 @@
>          bl    puts    ;     \
>          mov   lr, x3  ;     \
>          RODATA_STR(98, _s)
> +
> +/*
> + * Macro to print the value of register \xb
> + *
> + * Clobbers x0 - x4
> + */

Despite its name, this macro can't print x4. I would recommend adding at
least comment about this. Static assertion would be even better, but
looks like we don't have them for asm code.

> +.macro print_reg xb
> +        mov   x4, lr
> +        mov   x0, \xb
> +        bl    putn
> +        mov   lr, x4
> +.endm
> +
>  #else /* CONFIG_EARLY_PRINTK */
>  #define PRINT(s)
> +
> +.macro print_reg xb
> +.endm
> +
>  #endif /* !CONFIG_EARLY_PRINTK */
>
>  /* Load the physical address of a symbol into xb */
> @@ -304,22 +321,20 @@ GLOBAL(init_secondary)
>  #ifdef CONFIG_EARLY_PRINTK
>          ldr   x23, =EARLY_UART_BASE_ADDRESS /* x23 := UART base address */
>          PRINT("- CPU ")
> -        mov   x0, x24
> -        bl    putn
> +        print_reg x24
>          PRINT(" booting -\r\n")
>  #endif
>
>  common_start:
>
>          PRINT("- Current EL ")
> -        mrs   x4, CurrentEL
> -        mov   x0, x4
> -        bl    putn
> +        mrs   x5, CurrentEL
> +        print_reg x5
>          PRINT(" -\r\n")
>
>          /* Are we in EL2 */
> -        cmp   x4, #PSR_MODE_EL2t
> -        ccmp  x4, #PSR_MODE_EL2h, #0x4, ne
> +        cmp   x5, #PSR_MODE_EL2t
> +        ccmp  x5, #PSR_MODE_EL2h, #0x4, ne
>          b.eq  el2 /* Yes */
>
>          /* OK, we're boned. */

--
Best regards, Volodymyr Babchuk
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