[PATCH v2] KVM: arm/arm64: Access CNTHCTL_EL2 bit fields correctly

[Marc Zyngier]

Hi Jintack,

On 01/12/16 11:38, Jintack Lim wrote:
> Current KVM world switch code is unintentionally setting wrong bits to
> CNTHCTL_EL2 when E2H == 1, which may allow guest OS to access physical
> timer.  Bit positions of CNTHCTL_EL2 are changing depending on
> HCR_EL2.E2H bit.  EL1PCEN and EL1PCTEN are 1st and 0th bits when E2H is
> not set, but they are 11th and 10th bits respectively when E2H is set.
> 
> In fact, on VHE we only need to set those bits once, not for every world
> switch. This is because the host kernel runs in EL2 with HCR_EL2.TGE ==
> 1, which makes those bits have no effect for the host kernel execution.
> So we just set those bits once for guests, and that's it.
> 
> Signed-off-by: Jintack Lim <jintack at cs.columbia.edu>
> ---
> v2: Skip configuring cnthctl_el2 in world switch path on VHE system.
> This patch is based on linux-next.
> 
> ---
>  arch/arm/kvm/arm.c           |  1 +
>  include/kvm/arm_arch_timer.h |  1 +
>  virt/kvm/arm/arch_timer.c    | 23 ++++++++++++++++++++++
>  virt/kvm/arm/hyp/timer-sr.c  | 45 ++++++++++++++++++++++++++++++++------------
>  4 files changed, 58 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 8f92efa..38c0645 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -1286,6 +1286,7 @@ static void teardown_hyp_mode(void)
>  
>  static int init_vhe_mode(void)
>  {
> +	on_each_cpu(kvm_timer_init_vhe, NULL, 1);

I'm pretty sure this is not going to work with CPU hotplug, as you only
perform this once and for all at boot time.

I think it would be better if you called this init function as part of
cpu_init_hyp_mode().

>  	kvm_info("VHE mode initialized successfully\n");
>  	return 0;
>  }
> diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
> index dda39d8..5853399 100644
> --- a/include/kvm/arm_arch_timer.h
> +++ b/include/kvm/arm_arch_timer.h
> @@ -76,4 +76,5 @@ int kvm_timer_vcpu_reset(struct kvm_vcpu *vcpu,
>  
>  void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu);
>  
> +void kvm_timer_init_vhe(void *dummy);
>  #endif
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 17b8fa5..7a0d85d7 100644
> --- a/virt/kvm/arm/arch_timer.c
> +++ b/virt/kvm/arm/arch_timer.c
> @@ -24,6 +24,7 @@
>  
>  #include <clocksource/arm_arch_timer.h>
>  #include <asm/arch_timer.h>
> +#include <asm/kvm_hyp.h>
>  
>  #include <kvm/arm_vgic.h>
>  #include <kvm/arm_arch_timer.h>
> @@ -507,3 +508,25 @@ void kvm_timer_init(struct kvm *kvm)
>  {
>  	kvm->arch.timer.cntvoff = kvm_phys_timer_read();
>  }
> +
> +/*
> + * On VHE system, we only need to configure trap on physical timer and counter
> + * accesses in EL0 and EL1 once, not for every world switch.
> + * The host kernel runs at EL2 with HCR_EL2.TGE == 1,
> + * and this makes those bits have no effect for the host kernel execution.
> + */
> +void kvm_timer_init_vhe(void *dummy)
> +{
> +	/* When HCR_EL2.E2H ==1, EL1PCEN and EL1PCTEN are shifted by 10 */
> +	u32 cnthctl_shift = 10;
> +	u64 val;
> +
> +	/*
> +	 * Disallow physical timer access for the guest.
> +	 * Physical counter access is allowed.
> +	 */
> +	val = read_sysreg(cnthctl_el2);
> +	val &= ~(CNTHCTL_EL1PCEN << cnthctl_shift);
> +	val |= (CNTHCTL_EL1PCTEN << cnthctl_shift);
> +	write_sysreg(val, cnthctl_el2);
> +}

If you make this called from cpu_init_hyp_mode(), it will have to be
guarded with is_kernel_in_hyp_mode().

> diff --git a/virt/kvm/arm/hyp/timer-sr.c b/virt/kvm/arm/hyp/timer-sr.c
> index 798866a..f7fc957 100644
> --- a/virt/kvm/arm/hyp/timer-sr.c
> +++ b/virt/kvm/arm/hyp/timer-sr.c
> @@ -21,6 +21,18 @@
>  
>  #include <asm/kvm_hyp.h>
>  
> +#ifdef CONFIG_ARM64
> +static inline bool has_vhe(void)
> +{
> +	if (cpus_have_const_cap(ARM64_HAS_VIRT_HOST_EXTN))
> +		return true;
> +
> +	return false;
> +}
> +#else
> +#define has_vhe()	false

Could this now be moved to asm/virt.h, and maybe replace the current
implementation of is_kernel_in_hyp_mode? The latter may require some
more hacking around, so I'm not sure this is worth it.

> +#endif
> +
>  /* vcpu is already in the HYP VA space */
>  void __hyp_text __timer_save_state(struct kvm_vcpu *vcpu)
>  {
> @@ -35,10 +47,16 @@ void __hyp_text __timer_save_state(struct kvm_vcpu *vcpu)
>  	/* Disable the virtual timer */
>  	write_sysreg_el0(0, cntv_ctl);
>  
> -	/* Allow physical timer/counter access for the host */
> -	val = read_sysreg(cnthctl_el2);
> -	val |= CNTHCTL_EL1PCTEN | CNTHCTL_EL1PCEN;
> -	write_sysreg(val, cnthctl_el2);
> +	/*
> +	 * We don't need to do this for VHE since the host kernel runs in EL2
> +	 * with HCR_EL2.TGE ==1, which makes those bits have no impact.
> +	 */
> +	if (!has_vhe()) {
> +		/* Allow physical timer/counter access for the host */
> +		val = read_sysreg(cnthctl_el2);
> +		val |= CNTHCTL_EL1PCTEN | CNTHCTL_EL1PCEN;
> +		write_sysreg(val, cnthctl_el2);
> +	}
>  
>  	/* Clear cntvoff for the host */
>  	write_sysreg(0, cntvoff_el2);
> @@ -50,14 +68,17 @@ void __hyp_text __timer_restore_state(struct kvm_vcpu *vcpu)
>  	struct arch_timer_cpu *timer = &vcpu->arch.timer_cpu;
>  	u64 val;
>  
> -	/*
> -	 * Disallow physical timer access for the guest
> -	 * Physical counter access is allowed
> -	 */
> -	val = read_sysreg(cnthctl_el2);
> -	val &= ~CNTHCTL_EL1PCEN;
> -	val |= CNTHCTL_EL1PCTEN;
> -	write_sysreg(val, cnthctl_el2);
> +	/* Those bits are already configured at boot on VHE-system */
> +	if (!has_vhe()) {
> +		/*
> +		 * Disallow physical timer access for the guest
> +		 * Physical counter access is allowed
> +		 */
> +		val = read_sysreg(cnthctl_el2);
> +		val &= ~CNTHCTL_EL1PCEN;
> +		val |= CNTHCTL_EL1PCTEN;
> +		write_sysreg(val, cnthctl_el2);
> +	}
>  
>  	if (timer->enabled) {
>  		write_sysreg(kvm->arch.timer.cntvoff, cntvoff_el2);
> 

Otherwise, this looks good, and the generated code is quite nice.

Thanks,

	M.
-- 
Jazz is not dead. It just smells funny...