Subject: [PATCH 1/3] arm64: mm: Support Common Not Private translations

From: [email-masked] (James Morse)
Date: Tue, 10 Oct 2017 16:19:23 +0100

Hi Vladimir,

On 10/10/17 13:50, Vladimir Murzin wrote:
> On 09/10/17 17:48, James Morse wrote:
>> On 09/10/17 16:23, Catalin Marinas wrote:
>>> On Mon, Oct 09, 2017 at 01:55:32PM +0100, Vladimir Murzin wrote:
>>>> This patch adds support for Common Not Private translations on
>>>> different exceptions levels:
>>
>>>> (2) For EL1 we postpone setting CNP till all cpus are up and rely on
>>>>     cpufeature framework to 1) patch the code which is sensitive to
>>>>     CNP and 2) update TTBR1_EL1 with CNP bit set. The only case where
>>>>     TTBR1_EL1 can be reprogrammed is hibirnation, so the code there is
>>>>     changed to save raw TTBR1_EL1 and blindly restore it on resume.
>>
>>> Even if you do this when all the CPUs are up, that's not always true.
>>> Starting with maxcpus=1 allows something like systemd to bring up new
>>> CPUs once user space starts.
>>
>> For secondary CPUs cpu_enable_cnp() will be called to set CNP on TTBR1_EL1. But
>> what about cpuidle? This also resets the TTBR1_EL1 value.
> 
> Good point! I've missed it because reset happens via __enable_mmu, which has 
> no idea about CnP.
> 
> Would something like below be sufficient?
> 
> 
> diff --git a/arch/arm64/kernel/suspend.c b/arch/arm64/kernel/suspend.c
> index 1e3be90..03a02c4 100644
> --- a/arch/arm64/kernel/suspend.c
> +++ b/arch/arm64/kernel/suspend.c
> @@ -46,6 +46,10 @@ void notrace __cpu_suspend_exit(void)
>  	 */
>  	cpu_uninstall_idmap();
>  
> +#ifdef CONFIG_ARM64_CNP
> +	/* Restore CnP bit in TTBR1_EL1 */
> +	cpu_replace_ttbr1(swapper_pg_dir);
> +#endif
>  	/*
>  	 * PSTATE was not saved over suspend/resume, re-enable any detected
>  	 * features that might not have been set correctly.
> 

This re-install -> uninstalls the idmap, and if we don't actually have CNP
support, it wouldn't have changed anything. How about:

> if (cpus_have_const_cap(ARM64_HAS_CNP)
> 	cpu_replace_ttbr1(lm_alias(swapper_pg_dir));

We could look at having a combined helper that is called with the idmap
installed and does the uninstall.

hibernate uses these cpu_suspend helpers to save/restore the CPU registers, so
if we fix cpu-idle, you don't need the hibernate hunk anymore.

Thanks,

James