Re: [PATCH v14 4/5] KVM: arm64: Set PSTATE.EXLOCK when entering an exception

[Marc Zyngier <maz@kernel.org>]

On Sat, 05 Oct 2024 11:37:31 +0100,
Mark Brown <broonie@kernel.org> wrote:
> 
> As per DDI 0487 RWTXBY we need to manage PSTATE.EXLOCK when entering an
> exception, when the exception is entered from a lower EL the bit is cleared
> while if entering from the same EL it is set to GCSCR_ELx.EXLOCKEN.
> Implement this behaviour in enter_exception64().
> 
> Signed-off-by: Mark Brown <broonie@kernel.org>
> ---
>  arch/arm64/include/uapi/asm/ptrace.h |  2 ++
>  arch/arm64/kvm/hyp/exception.c       | 10 ++++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
> index 0f39ba4f3efd4a8760f0fca0fbf1a2563b191c7d..9987957f4f7137bf107653b817885bb976853a83 100644
> --- a/arch/arm64/include/uapi/asm/ptrace.h
> +++ b/arch/arm64/include/uapi/asm/ptrace.h
> @@ -37,6 +37,7 @@
>  #define PSR_MODE_EL3t	0x0000000c
>  #define PSR_MODE_EL3h	0x0000000d
>  #define PSR_MODE_MASK	0x0000000f
> +#define PSR_EL_MASK	0x0000000c
>  
>  /* AArch32 CPSR bits */
>  #define PSR_MODE32_BIT		0x00000010
> @@ -56,6 +57,7 @@
>  #define PSR_C_BIT	0x20000000
>  #define PSR_Z_BIT	0x40000000
>  #define PSR_N_BIT	0x80000000
> +#define PSR_EXLOCK_BIT 0x400000000
>  
>  #define PSR_BTYPE_SHIFT		10
>  
> diff --git a/arch/arm64/kvm/hyp/exception.c b/arch/arm64/kvm/hyp/exception.c
> index 424a5107cddb5e1cdd75ef3581adef03aaadabb7..0d41b9b75cf83250b2c0d20cd82c153869efb0e4 100644
> --- a/arch/arm64/kvm/hyp/exception.c
> +++ b/arch/arm64/kvm/hyp/exception.c
> @@ -160,6 +160,16 @@ static void enter_exception64(struct kvm_vcpu *vcpu, unsigned long target_mode,
>  	// PSTATE.BTYPE is set to zero upon any exception to AArch64
>  	// See ARM DDI 0487E.a, pages D1-2293 to D1-2294.
>  
> +	// PSTATE.EXLOCK is set to 0 upon any exception to a higher
> +	// EL, or to GCSCR_ELx.EXLOCKEN for an exception to the same
> +	// exception level.  See ARM DDI 0487 RWTXBY, D.1.3.2 in K.a.
> +	if (kvm_has_gcs(vcpu->kvm) &&
> +	    (target_mode & PSR_EL_MASK) == (mode & PSR_EL_MASK)) {
> +		u64 gcscr = __vcpu_read_sys_reg(vcpu, GCSCR_EL1);

No, please. This only works by luck when a guest has AArch32 EL0, and
creates more havoc on a NV guest. In general, this PSR_EL_MASK creates
more problem than anything else, and doesn't fit the rest of the code.

So this needs to:
- explicitly only apply to exceptions from AArch64
- handle exception from EL2, since this helper already deals with that

The latter point of course means introducing GCSCR_EL2 (and everything
that depends on it, such as the trap handling).

	M.

-- 
Without deviation from the norm, progress is not possible.