On Wed, 20 Aug 2025 15:14:44 +0100,
Mark Brown <broonie@xxxxxxxxxx> wrote:
>
> As per DDI 0487 RWTXBY we need to manage PSTATE.EXLOCK when entering an
Nit: please use an underscore between the type of a statement and its
"name", as it makes it a bit more readable (R_WTXBY).
> 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@xxxxxxxxxx>
> ---
> arch/arm64/include/uapi/asm/ptrace.h | 1 +
> arch/arm64/kvm/hyp/exception.c | 37 ++++++++++++++++++++++++++++++++++++
> 2 files changed, 38 insertions(+)
>
> diff --git a/arch/arm64/include/uapi/asm/ptrace.h b/arch/arm64/include/uapi/asm/ptrace.h
> index 0f39ba4f3efd..f2fb029fb61a 100644
> --- a/arch/arm64/include/uapi/asm/ptrace.h
> +++ b/arch/arm64/include/uapi/asm/ptrace.h
> @@ -56,6 +56,7 @@
> #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 95d186e0bf54..46e1d0c3038c 100644
> --- a/arch/arm64/kvm/hyp/exception.c
> +++ b/arch/arm64/kvm/hyp/exception.c
> @@ -73,6 +73,38 @@ static void __vcpu_write_spsr_und(struct kvm_vcpu *vcpu, u64 val)
> vcpu->arch.ctxt.spsr_und = val;
> }
>
> +static unsigned long enter_exception64_gcs(struct kvm_vcpu *vcpu,
> + unsigned long mode,
> + unsigned long target_mode)
A more appropriate name would be compute_exlock().
> +{
> + u64 gcscr;
> +
> + if (!kvm_has_gcs(kern_hyp_va(vcpu->kvm)))
> + return 0;
> +
> + /* GCS can't be enabled for 32 bit */
> + if (mode & PSR_MODE32_BIT)
> + return 0;
> +
> + /* When taking an exception to a higher EL EXLOCK is cleared. */
> + if ((mode | PSR_MODE_THREAD_BIT) != target_mode)
> + return 0;
> +
> + /*
> + * When taking an exception to the same EL EXLOCK is set to
> + * the effective value of GCSR_ELx.EXLOCKEN.
> + */
> + if (vcpu_is_el2(vcpu))
> + gcscr = __vcpu_read_sys_reg(vcpu, GCSCR_EL2);
> + else
> + gcscr = __vcpu_read_sys_reg(vcpu, GCSCR_EL1);
> +
> + if (gcscr & GCSCR_ELx_EXLOCKEN)
> + return PSR_EXLOCK_BIT;
> +
> + return 0;
> +}
> +
> /*
> * This performs the exception entry at a given EL (@target_mode), stashing PC
> * and PSTATE into ELR and SPSR respectively, and compute the new PC/PSTATE.
> @@ -162,6 +194,11 @@ 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.
> + new |= enter_exception64_gcs(vcpu, mode, target_mode);
> +
> new |= PSR_D_BIT;
> new |= PSR_A_BIT;
> new |= PSR_I_BIT;
>
But that's not the only case where we have to deal with EXLOCK, is it?
What of ERET and its PAuth variants? R_TYTWB says:
<quote>
If in AArch64 state, any of the following situations can cause an
illegal exception return:
[...]
- If the Effective value of GCSCR_ELx.EXLOCKEN is 1 and PSTATE.EXLOCK
is 0, the execution of an exception return instruction to return to
the current Exception level ELx.
</quote>
My reading of the spec is that this needs handling.
M.
--
Jazz isn't dead. It just smells funny.
