linux-kernel - Re: [PATCHv2] arm: LPAE: kexec: clear TTBCR.T1SZ explicitly when PHYS_OFFSET > PAGE

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Date:   Mon, 5 Jun 2017 20:06:47 +0900
From:   Hoeun Ryu <hoeun.ryu@...il.com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Russell King <linux@...linux.org.uk>, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv2] arm: LPAE: kexec: clear TTBCR.T1SZ explicitly when PHYS_OFFSET > PAGE_OFFSET


>> On Jun 5, 2017, at 7:30 PM, Robin Murphy <robin.murphy@....com> wrote:
>> 
>> On 05/06/17 11:06, Hoeun Ryu wrote:
>> Clearing TTBCR.T1SZ explicitly when kernel runs on a configuration of
>> PHYS_OFFSET > PAGE_OFFSET.
>> Reading TTBCR in early boot stage might returns the value of the
>> previous kernel's configuration, especially in case of kexec. For
>> example, if normal kernel (first kernel) had run on a configuration
>> of PHYS_OFFSET <= PAGE_OFFSET and crash kernel (second kernel) is
>> running on a configuration PHYS_OFFSET > PAGE_OFFSET, which can happen
>> because it depends on the reserved area for crash kernel, reading
>> TTBCR and using the value without clearing TTBCR.T1SZ might risky
>> because the value doesn't have a reset value for TTBCR.T1SZ.
> 
> Hmm, but isn't that also strictly true of all the other fields we're
> ORing TTB_FLAGS_* into? Given that this is initial setup, I wonder
> whether we need to care about the previous TTBCR value at all - is there
> any reason for not simply building up the value we want from scratch?

Your idea seems better.
I'll send a new patch to build TTBCR from scratch without reading it for the initial value.

Russell, do you have any opinion ?

Thank toy for the review.

> 
> Robin.
> 
>> Signed-off-by: Hoeun Ryu <hoeun.ryu@...il.com>
>> ---
>> arch/arm/mm/proc-v7-3level.S | 1 +
>> 1 file changed, 1 insertion(+)
>> 
>> diff --git a/arch/arm/mm/proc-v7-3level.S b/arch/arm/mm/proc-v7-3level.S
>> index 5e5720e..81404b8 100644
>> --- a/arch/arm/mm/proc-v7-3level.S
>> +++ b/arch/arm/mm/proc-v7-3level.S
>> @@ -140,6 +140,7 @@ ENDPROC(cpu_v7_set_pte_ext)
>>    * otherwise booting secondary CPUs would end up using TTBR1 for the
>>    * identity mapping set up in TTBR0.
>>    */
>> +    bichi    \tmp, \tmp, #(7 << 16)                @ clear TTBCR.T1SZ
>>   orrls    \tmp, \tmp, #TTBR1_SIZE                @ TTBCR.T1SZ
>>   mcr    p15, 0, \tmp, c2, c0, 2                @ TTBCR
>>   mov    \tmp, \ttbr1, lsr #20
>