[PATCH] efi-entry.S: add dsb and isb

[Tom Lendacky]

On 11/12/2014 10:58 AM, Mark Rutland wrote:
> Hi Joel,
>
> I've Cc'd a few people who were involved in authoring this.
>
> I have an alternative patch [1] that also adds some missing maintenance.
> Would you be able to give that a go?

I applied the referenced patch and have booted about two dozen times
without seeing a synchronous exception.  Prior to either patch I would
see a synchronous exception about every three or four (re)boots.

Thanks,
Tom

>
> On Tue, Nov 11, 2014 at 06:32:04AM +0000, Joel Schopp wrote:
>> Add a dsb and isb after the instruction flush before the data cache and
>> mm offing.  Without this patch I am seeing synchronous exceptions occur
>> every few boots.
>>
>> Signed-off-by: Joel Schopp <joel.schopp at amd.com>
>> Tested-by: Tom Lendacky <Thomas.Lendacky at amd.com>
>> ---
>>   arch/arm64/kernel/efi-entry.S |    6 ++++++
>>   1 file changed, 6 insertions(+)
>>
>> diff --git a/arch/arm64/kernel/efi-entry.S b/arch/arm64/kernel/efi-entry.S
>> index 619b1dd..7d95eda 100644
>> --- a/arch/arm64/kernel/efi-entry.S
>> +++ b/arch/arm64/kernel/efi-entry.S
>> @@ -76,6 +76,12 @@ ENTRY(efi_stub_entry)
>>   	bl	__flush_dcache_area
>>   	ic	ialluis
>>
>> +	/* We need to sync again after the instruction cache sync
>> +	 * and before turning off the dcache and mmu
>> +	 */
>> +	dsb	sy
>> +	isb
>
> In my series I reasoned that it wasn't necessary to have an ISB before
> we disabled the MMU. The current image must already be visible to the
> I-cache, so the I-cache can't have stale entries for it. We don't
> disable the MMU until after the image is visible at the PoC, so we
> shouldn't break the visbility of the current image to the I-cache.
>
> There is a bug in that We don't flush the current image in case of
> relocation, but I don't see how the ISB would help there.
>
> Thanks,
> Mark.
>
>> +
>>   	/* Turn off Dcache and MMU */
>>   	mrs	x0, CurrentEL
>>   	cmp	x0, #CurrentEL_EL2
>>
>>
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>>
>
> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2014-November/301779.html
>