Re: [PATCH 26/26] KVM: arm64: Parametrize exception entry with a target EL

Mark Rutland
2020-05-19T10:45:11Z

On Wed, Apr 22, 2020 at 01:00:50PM +0100, Marc Zyngier wrote:
> We currently assume that an exception is delivered to EL1, always.
> Once we emulate EL2, this no longer will be the case. To prepare
> for this, add a target_mode parameter.
>
> While we're at it, merge the computing of the target PC and PSTATE in
> a single function that updates both PC and CPSR after saving their
> previous values in the corresponding ELR/SPSR. This ensures that they
> are updated in the correct order (a pretty common source of bugs...).
>
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
> arch/arm64/kvm/inject_fault.c | 75 ++++++++++++++++++-----------------
> 1 file changed, 38 insertions(+), 37 deletions(-)
>
> diff --git a/arch/arm64/kvm/inject_fault.c b/arch/arm64/kvm/inject_fault.c
> index d3ebf8bca4b89..3dbcbc839b9c3 100644
> --- a/arch/arm64/kvm/inject_fault.c
> +++ b/arch/arm64/kvm/inject_fault.c
> @@ -26,28 +26,12 @@ enum exception_type {
> except_type_serror = 0x180,
> };
>
> -static u64 get_except_vector(struct kvm_vcpu *vcpu, enum exception_type type)
> -{
> - u64 exc_offset;
> -
> - switch (*vcpu_cpsr(vcpu) & (PSR_MODE_MASK | PSR_MODE32_BIT)) {
> - case PSR_MODE_EL1t:
> - exc_offset = CURRENT_EL_SP_EL0_VECTOR;
> - break;
> - case PSR_MODE_EL1h:
> - exc_offset = CURRENT_EL_SP_ELx_VECTOR;
> - break;
> - case PSR_MODE_EL0t:
> - exc_offset = LOWER_EL_AArch64_VECTOR;
> - break;
> - default:
> - exc_offset = LOWER_EL_AArch32_VECTOR;
> - }
> -
> - return vcpu_read_sys_reg(vcpu, VBAR_EL1) + exc_offset + type;
> -}
> -
> /*
> + * 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.
> + * The EL passed to this function *must* be a non-secure, privileged mode with
> + * bit 0 being set (PSTATE.SP == 1).
> + *
> * When an exception is taken, most PSTATE fields are left unchanged in the
> * handler. However, some are explicitly overridden (e.g. M[4:0]). Luckily all
> * of the inherited bits have the same position in the AArch64/AArch32 SPSR_ELx
> @@ -59,10 +43,35 @@ static u64 get_except_vector(struct kvm_vcpu *vcpu, enum exception_type type)
> * Here we manipulate the fields in order of the AArch64 SPSR_ELx layout, from
> * MSB to LSB.
> */
> -static unsigned long get_except64_pstate(struct kvm_vcpu *vcpu)
> +static void enter_exception(struct kvm_vcpu *vcpu, unsigned long target_mode,
> + enum exception_type type)

Since this is all for an AArch64 target, could we keep `64` in the name,
e.g enter_exception64? That'd mirror the callers below.

> {
> - unsigned long sctlr = vcpu_read_sys_reg(vcpu, SCTLR_EL1);
> - unsigned long old, new;
> + unsigned long sctlr, vbar, old, new, mode;
> + u64 exc_offset;
> +
> + mode = *vcpu_cpsr(vcpu) & (PSR_MODE_MASK | PSR_MODE32_BIT);
> +
> + if (mode == target_mode)
> + exc_offset = CURRENT_EL_SP_ELx_VECTOR;
> + else if ((mode | 1) == target_mode)
> + exc_offset = CURRENT_EL_SP_EL0_VECTOR;

It would be nice if we could add a mnemonic for the `1` here, e.g.
PSR_MODE_SP0 or PSR_MODE_THREAD_BIT.

> + else if (!(mode & PSR_MODE32_BIT))
> + exc_offset = LOWER_EL_AArch64_VECTOR;
> + else
> + exc_offset = LOWER_EL_AArch32_VECTOR;

Other than the above, I couldn't think of a nicer way of writing thism
and AFAICT this is correct.

> +
> + switch (target_mode) {
> + case PSR_MODE_EL1h:
> + vbar = vcpu_read_sys_reg(vcpu, VBAR_EL1);
> + sctlr = vcpu_read_sys_reg(vcpu, SCTLR_EL1);
> + vcpu_write_sys_reg(vcpu, *vcpu_pc(vcpu), ELR_EL1);
> + break;
> + default:
> + /* Don't do that */
> + BUG();
> + }
> +
> + *vcpu_pc(vcpu) = vbar + exc_offset + type;
>
> old = *vcpu_cpsr(vcpu);
> new = 0;
> @@ -105,9 +114,10 @@ static unsigned long get_except64_pstate(struct kvm_vcpu *vcpu)
> new |= PSR_I_BIT;
> new |= PSR_F_BIT;
>
> - new |= PSR_MODE_EL1h;
> + new |= target_mode;

As a heads-up, some of the other bits will need to change for an EL2
target (e.g. SPAN will depend on HCR_EL2.E2H), but as-is this this is
fine.

Regardless of the above comments:

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

Mark.