Re: [PATCH] arm64/relocate_kernel: remove redundant but misleading code

[James Morse <james.morse@arm.com>]

Hi Liu,

On 06/08/2020 09:26, Pingfan Liu wrote:
> The kexec switch sequence looks like the following:
>   SYM_CODE_START(__cpu_soft_restart)
>     ...
>     pre_disable_mmu_workaround
>     msr	sctlr_el1, x12
>     ...
>     br	x8
> 
>   SYM_CODE_START(arm64_relocate_new_kernel)
>     ...
>     pre_disable_mmu_workaround
>     msr	sctlr_el2, x0
>     ...

> "msr sctlr_el2, x0" is misleading, because "br x8" jump to a physical
> address, which has no entry in idmap.

Even better: this code run from a copy allocated by kexec, its not in the idmap either.

See the memcpy() in machine_kexec().


> It implies that MMU has already been fully off after "msr sctlr_el1, x12".


> And according to "D12.2.101 SCTLR_EL2, System Control Register (EL2)" in
> "ARM Architecture Reference Manual", actually, EL2&0 host accesses
> to SCTLR_EL2 when using mnemonic SCTLR_EL1.

Only when HCR_EL2.E2H is enabled. If linux booted at EL2 on a non-VHE system, SCTLR_EL1
and SCTLR_EL2 are different registers, both of which are managed by linux/KVM.


> Hence removing the redundant but misleading code.

This isn't the reason its redundant...


> diff --git a/arch/arm64/kernel/cpu-reset.S b/arch/arm64/kernel/cpu-reset.S
> index 4a18055..37721eb 100644
> --- a/arch/arm64/kernel/cpu-reset.S
> +++ b/arch/arm64/kernel/cpu-reset.S
> @@ -35,6 +35,10 @@ SYM_CODE_START(__cpu_soft_restart)
>  	mov_q	x13, SCTLR_ELx_FLAGS
>  	bic	x12, x12, x13
>  	pre_disable_mmu_workaround
> +	/*
> +	 * either disable EL1&0 translation regime or disable EL2&0 translation
> +	 * regime if HCR_EL2.E2H == 1
> +	 */>  	msr	sctlr_el1, x12
>  	isb

On a VHE system, yes the cpu-reset.S disables EL2&0 by writing to SCTLR_EL1

But on a non-VHE system, that same code disabled EL1&0. cup-reset.S goes on to call
HVC_SOFT_RESTART for EL2, which may be serviced by KVM or the hyp-stub. (or maybe
something else that implements the hyp-stub api)

For kexec, on non-VHE both EL1&0 and EL2 get disabled.


> diff --git a/arch/arm64/kernel/relocate_kernel.S b/arch/arm64/kernel/relocate_kernel.S
> index 542d6ed..84eec95 100644
> --- a/arch/arm64/kernel/relocate_kernel.S
> +++ b/arch/arm64/kernel/relocate_kernel.S
> @@ -36,18 +36,6 @@ SYM_CODE_START(arm64_relocate_new_kernel)
>  	mov	x14, xzr			/* x14 = entry ptr */
>  	mov	x13, xzr			/* x13 = copy dest */
>  
> -	/* Clear the sctlr_el2 flags. */
> -	mrs	x0, CurrentEL
> -	cmp	x0, #CurrentEL_EL2
> -	b.ne	1f
> -	mrs	x0, sctlr_el2
> -	mov_q	x1, SCTLR_ELx_FLAGS
> -	bic	x0, x0, x1
> -	pre_disable_mmu_workaround
> -	msr	sctlr_el2, x0
> -	isb
> -1:

I agree this doesn't disable the MMU anymore.

This was originally kept to disable the I+C bits when Kdump interrupted KVM, but since KVM
formalised the hyp-stub API, and has this exact sequence to back its HVC_SOFT_RESTART, it
was only needed for the hyp-stub itself, which has no clue about these SCTLR_EL2 bits.

HVC_SOFT_RESTART only says it needs to disable the MMU. See
Documentation/virt/kvm/arm/hyp-abi.rst


I think its fine to remove this, but the reason is because el2_setup doesn't set those
bits, and KVM clears them when its unloaded, or has a HVC_SOFT_RESTART call.

It might be worth updating the document, but we'd need to check the guarantee is the same
on 32bit. I assume there is no out-of-tree user of the hyp-stub abi.


I don't think the E2H register redirection has anything to do with this.



Thanks,

James

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