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 14:17:54 +0800
From:   Tiezhu Yang <yangtiezhu@...ngson.cn>
To:     Jiaxun Yang <jiaxun.yang@...goat.com>,
        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

On 11/03/2020 01:28 PM, Jiaxun Yang wrote:
>
>
> 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

Hi Jiaxun,

Thanks for your detail review, it looks better.
Let me update it and then send v3.

Thanks,
Tiezhu

>
> ---
> 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"