linux-kernel - Re: [PATCH v2 5/6] MIPS: Loongson64: Make sure the PC address is correct when 3A4000+ CPU hotplug

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Date:   Tue, 3 Nov 2020 13:28:39 +0800
From:   Jiaxun Yang <jiaxun.yang@...goat.com>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        Huacai Chen <chenhc@...ote.com>
Cc:     linux-mips@...r.kernel.org, linux-kernel@...r.kernel.org,
        Xuefeng Li <lixuefeng@...ngson.cn>,
        Lu Zeng <zenglu@...ngson.cn>, Jun Yi <yijun@...ngson.cn>
Subject: Re: [PATCH v2 5/6] MIPS: Loongson64: Make sure the PC address is
 correct when 3A4000+ CPU hotplug



在 2020/11/3 11:15, Tiezhu Yang 写道:
> In loongson3_type3_play_dead(), in order to make sure the PC address is
> correct, use lw to read the low 32 bits first, if the result is not zero,
> then use ld to read the whole 64 bits, otherwise there maybe exists atomic
> problem due to write high 32 bits first and then low 32 bits, like this:
>
> high 32 bits (write done)
>                                    -- only read high 32-bits which is wrong
> low 32 bits (not yet write done)
>
> This problem is especially for Loongson 3A4000+ CPU due to using Mail_Send
> register which can only send 32 bits data one time. Although it is hard to
> reproduce, we can do something at the software level to avoid the risks for
> 3A4000+ CPU, this change has no influence on the other Loongson CPUs.
>
> Signed-off-by: Lu Zeng <zenglu@...ngson.cn>
> Signed-off-by: Jun Yi <yijun@...ngson.cn>
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>

Hi Tiezhu,

Sorry that I didn't look this patch carefully in previous rev, here's my 
comments,

Firstly the commit message and code comment looks bogus...

I'd prefer

---
MIPS: Loongson64: SMP: Fix up play_dead jump indicator

In play_dead function, the whole 64-bit PC mailbox was used as a indicator
to determine if the master core had written boot jump information.

However, after we introduced CSR mailsend, the 64-bit PC mailbox won't be
written atomicly. Thus we have to use the lower 32-bit, which will be 
written at
the last, as the jump indicator instead.
--

Thanks.

> ---
>
> v2: No changes
>
>   arch/mips/loongson64/smp.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/mips/loongson64/smp.c b/arch/mips/loongson64/smp.c
> index 736e98d..e32b46e 100644
> --- a/arch/mips/loongson64/smp.c
> +++ b/arch/mips/loongson64/smp.c
> @@ -764,9 +764,10 @@ static void loongson3_type3_play_dead(int *state_addr)
>   		"1: li    %[count], 0x100             \n" /* wait for init loop */
>   		"2: bnez  %[count], 2b                \n" /* limit mailbox access */
>   		"   addiu %[count], -1                \n"
> -		"   ld    %[initfunc], 0x20(%[base])  \n" /* get PC via mailbox */
> +		"   lw    %[initfunc], 0x20(%[base])  \n" /* get PC (low 32 bits) via mailbox */

Here you can comment as "Check jump indicator (lower 32-bit of PC mailbox)"

Thanks.

- Jiaxun
>   		"   beqz  %[initfunc], 1b             \n"
>   		"   nop                               \n"
> +		"   ld    %[initfunc], 0x20(%[base])  \n" /* get PC (whole 64 bits) via mailbox */
>   		"   ld    $sp, 0x28(%[base])          \n" /* get SP via mailbox */
>   		"   ld    $gp, 0x30(%[base])          \n" /* get GP via mailbox */
>   		"   ld    $a1, 0x38(%[base])          \n"