Re: [PATCH] KVM: arm64: Don't populate TPIDR_EL2 in finalise_el2()

[Mark Rutland <mark.rutland@arm.com>]

On Mon, Mar 30, 2026 at 04:29:26PM +0100, Will Deacon wrote:
> When running with VHE, TPIDR_EL2 is only used for the percpu offset once
> the ARM64_HAS_VIRT_HOST_EXTN capability has been detected, at which
> point cpu_copy_el2regs() will populate its contents from TPIDR_EL1.
> 
> Remove the redundant initialisation of TPIDR_EL2 from finalise_el2().

It wasn't clear to me how this worked for both the boot CPU and
secondaries, since they both call finalise_el2(). How about a slightly
more elaborate commit message, e.g.

| Currently, it is not necessary for __finalise_el2() to configure
| TPIDR_EL2:
|
| * The hyp stub code does not consume the value of TPIDR_EL2.
|
| * On the boot cpu, TPIDR_EL1 is used for the percpu offset until the
|   ARM64_HAS_VIRT_HOST_EXTN cpucap is detected and boot alternatives
|   are patched. Before boot alternatives are patched,
|   cpu_copy_el2regs() will copy TPIDR_EL1 into TPIDR_EL2. It is not
|   necessary for __finalise_el2() to initialise TPIDR_EL2 before this.
|
| * Secondary CPUs are brought up after boot alternatives have been
|   patched, and __secondary_switched() will initialize TPIDR_EL2 in
|   'init_cpu_task', after finalise_el2() calls __finalise_el2()
|
| * KVM hyp code which may consume TPIDR_EL2 is brought up after all
|   secondaries have been booted, once TPIDR_El2 has been configured on
|   all CPUs.
|
| Remove the redundant initialisation from __finalise_el2().

> Cc: Oliver Upton <oupton@kernel.org>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Signed-off-by: Will Deacon <will@kernel.org>

Reviewed-by: Mark Rutland <mark.rutland@arm.com>

Mark.

> ---
> 
> I spotted this by inspection as part of an ill-fated (abandoned) attempt
> at repurposing tpidr_elx for something else.
> 
>  arch/arm64/kernel/hyp-stub.S | 4 +---
>  1 file changed, 1 insertion(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kernel/hyp-stub.S b/arch/arm64/kernel/hyp-stub.S
> index 085bc9972f6b..8b8614c0b9a5 100644
> --- a/arch/arm64/kernel/hyp-stub.S
> +++ b/arch/arm64/kernel/hyp-stub.S
> @@ -105,11 +105,9 @@ SYM_CODE_START_LOCAL(__finalise_el2)
>  	msr_hcr_el2 x0
>  	isb
>  
> -	// Use the EL1 allocated stack, per-cpu offset
> +	// Use the EL1 allocated stack
>  	mrs	x0, sp_el1
>  	mov	sp, x0
> -	mrs	x0, tpidr_el1
> -	msr	tpidr_el2, x0
>  
>  	// FP configuration, vectors
>  	mrs_s	x0, SYS_CPACR_EL12
> -- 
> 2.53.0.1018.g2bb0e51243-goog
>