[RFC PATCH v2 14/38] KVM: arm64: Synchronize EL1 system registers on virtual EL2 entry and exit

Christoffer Dall
2017-07-30T20:00:00Z

On Tue, Jul 18, 2017 at 11:58:40AM -0500, Jintack Lim wrote:
> When running in virtual EL2 we use the shadow EL1 systerm register array
> for the save/restore process, so that hardware and especially the memory
> subsystem behaves as code written for EL2 expects while really running
> in EL1.
>
> This works great for EL1 system register accesses that we trap, because
> these accesses will be written into the virtual state for the EL1 system
> registers used when eventually switching the VCPU mode to EL1.
>
> However, there was a collection of EL1 system registers which we do not
> trap, and as a consequence all save/restore operations of these
> registers were happening locally in the shadow array, with no benefit to
> software actually running in virtual EL1 at all.
>
> To fix this, simply synchronize the shadow and real EL1 state for these
> registers on entry/exit to/from virtual EL2 state.
>
> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> Signed-off-by: Jintack Lim <jintack.lim@linaro.org>
> ---
> arch/arm64/kvm/context.c | 58 ++++++++++++++++++++++++++++++++++++++++++++++--
> 1 file changed, 56 insertions(+), 2 deletions(-)
>
> diff --git a/arch/arm64/kvm/context.c b/arch/arm64/kvm/context.c
> index e965049..e1bc753 100644
> --- a/arch/arm64/kvm/context.c
> +++ b/arch/arm64/kvm/context.c
> @@ -86,6 +86,58 @@ static void flush_shadow_el1_sysregs(struct kvm_vcpu *vcpu)
> s_sys_regs[CPACR_EL1] = cptr_to_cpacr(vcpu_sys_reg(vcpu, CPTR_EL2));
> }
>
> +
> +/*
> + * List of EL0 and EL1 registers which we allow the virtual EL2 mode to access
> + * directly without trapping. This is possible because the impact of
> + * accessing those registers are the same regardless of the exception
> + * levels that are allowed.

I don't understand this last sentence...

> + */
> +static const int el1_non_trap_regs[] = {
> + CNTKCTL_EL1,
> + CSSELR_EL1,
> + PAR_EL1,
> + TPIDR_EL0,
> + TPIDR_EL1,
> + TPIDRRO_EL0
> +};
> +
> +/**
> + * copy_shadow_non_trap_el1_state
> + * @vcpu: The VCPU pointer
> + * @setup: True, if on the way to the guest (called from setup)

should setup be called flush?

then we could do

if (flush) {
...
} else { /* sync */
...
}

> + * False, if returning form the guet (calld from restore)

called

> + *
> + * Some EL1 registers are accessed directly by the virtual EL2 mode because
> + * they in no way affect execution state in virtual EL2. However, we must
> + * still ensure that virtual EL2 observes the same state of the EL1 registers
> + * as the normal VM's EL1 mode, so copy this state as needed on setup/restore.
> + */

Perhaps this could be written more clearly as:

/*
* Synchronize the state of EL1 registers directly accessible by virtual
* EL2 between the shadow sys_regs array and the VCPU's EL1 state
* before/after the world switch code copies the shadow state to/from
* hardware registers.
*/

> +static void copy_shadow_non_trap_el1_state(struct kvm_vcpu *vcpu, bool setup)
> +{
> + u64 *s_sys_regs = vcpu->arch.ctxt.shadow_sys_regs;
> + int i;
> +
> + for (i = 0; i < ARRAY_SIZE(el1_non_trap_regs); i++) {
> + const int sr = el1_non_trap_regs[i];
> +
> + if (setup)
> + s_sys_regs[sr] = vcpu_sys_reg(vcpu, sr);
> + else
> + vcpu_sys_reg(vcpu, sr) = s_sys_regs[sr];
> + }
> +}
> +
> +static void sync_shadow_non_trap_el1_state(struct kvm_vcpu *vcpu)
> +{
> + copy_shadow_non_trap_el1_state(vcpu, false);
> +}
> +
> +static void flush_shadow_non_trap_el1_state(struct kvm_vcpu *vcpu)
> +{
> + copy_shadow_non_trap_el1_state(vcpu, true);
> +}
> +
> static void flush_shadow_special_regs(struct kvm_vcpu *vcpu)
> {
> struct kvm_cpu_context *ctxt = &vcpu->arch.ctxt;
> @@ -162,6 +214,7 @@ void kvm_arm_setup_shadow_state(struct kvm_vcpu *vcpu)
> if (unlikely(vcpu_mode_el2(vcpu))) {
> flush_shadow_special_regs(vcpu);
> flush_shadow_el1_sysregs(vcpu);
> + flush_shadow_non_trap_el1_state(vcpu);
> ctxt->hw_sys_regs = ctxt->shadow_sys_regs;
> } else {
> flush_special_regs(vcpu);
> @@ -176,9 +229,10 @@ void kvm_arm_setup_shadow_state(struct kvm_vcpu *vcpu)
> */
> void kvm_arm_restore_shadow_state(struct kvm_vcpu *vcpu)
> {
> - if (unlikely(vcpu_mode_el2(vcpu)))
> + if (unlikely(vcpu_mode_el2(vcpu))) {
> sync_shadow_special_regs(vcpu);
> - else
> + sync_shadow_non_trap_el1_state(vcpu);
> + } else
> sync_special_regs(vcpu);
> }
>
> --
> 1.9.1
>