Subject: [PATCH 2/3] KVM: arm64: Access CNTHCTL_EL2 bit fields correctly on VHE systems

From: [email-masked] (Christoffer Dall)
Date: Fri, 13 Jan 2017 13:36:12 +0100

On Fri, Jan 13, 2017 at 11:31:32AM +0000, Marc Zyngier wrote:
> From: Jintack Lim <[email-masked]>
> 
> 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 <[email-masked]>
> Reviewed-by: Marc Zyngier <[email-masked]>
> Signed-off-by: Marc Zyngier <[email-masked]>
> ---
>  arch/arm/include/asm/virt.h   |  5 +++++
>  arch/arm/kvm/arm.c            |  3 +++
>  arch/arm64/include/asm/virt.h |  9 +++++++++
>  include/kvm/arm_arch_timer.h  |  1 +
>  virt/kvm/arm/arch_timer.c     | 23 +++++++++++++++++++++++
>  virt/kvm/arm/hyp/timer-sr.c   | 33 +++++++++++++++++++++------------
>  6 files changed, 62 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/arm/include/asm/virt.h b/arch/arm/include/asm/virt.h
> index a2e75b8..6dae195 100644
> --- a/arch/arm/include/asm/virt.h
> +++ b/arch/arm/include/asm/virt.h
> @@ -80,6 +80,11 @@ static inline bool is_kernel_in_hyp_mode(void)
>  	return false;
>  }
>  
> +static inline bool has_vhe(void)
> +{
> +	return false;
> +}
> +
>  /* The section containing the hypervisor idmap text */
>  extern char __hyp_idmap_text_start[];
>  extern char __hyp_idmap_text_end[];
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index 1167678..9d74464 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -1099,6 +1099,9 @@ static void cpu_init_hyp_mode(void *dummy)
>  	__cpu_init_hyp_mode(pgd_ptr, hyp_stack_ptr, vector_ptr);
>  	__cpu_init_stage2();
>  
> +	if (is_kernel_in_hyp_mode())
> +		kvm_timer_init_vhe();
> +
>  	kvm_arm_init_debug();
>  }
>  
> diff --git a/arch/arm64/include/asm/virt.h b/arch/arm64/include/asm/virt.h
> index fea1073..439f6b5 100644
> --- a/arch/arm64/include/asm/virt.h
> +++ b/arch/arm64/include/asm/virt.h
> @@ -47,6 +47,7 @@
>  #include <asm/ptrace.h>
>  #include <asm/sections.h>
>  #include <asm/sysreg.h>
> +#include <asm/cpufeature.h>
>  
>  /*
>   * __boot_cpu_mode records what mode CPUs were booted in.
> @@ -80,6 +81,14 @@ static inline bool is_kernel_in_hyp_mode(void)
>  	return read_sysreg(CurrentEL) == CurrentEL_EL2;
>  }
>  
> +static inline bool has_vhe(void)
> +{
> +	if (cpus_have_const_cap(ARM64_HAS_VIRT_HOST_EXTN))
> +		return true;
> +
> +	return false;
> +}
> +

I was experimenting with using has_vhe for some of the optimization code
I was writing, and I saw a hyp crash as a result.  That made me wonder
if this is really safe in Hyp mode?

Specifically, there is no guarantee that this will actually be inlined
in the caller, right?  At least that's what I can gather from trying to
understand the semantics of the inline keyword in the GCC manual.

Further, are we guaranteed that the static branch gets compiled into
something that doesn't actually look at cpu_hwcap_keys, which is not
mapped in hyp mode?

Thanks,
-Christoffer

>  #ifdef CONFIG_ARM64_VHE
>  extern void verify_cpu_run_el(void);
>  #else
> diff --git a/include/kvm/arm_arch_timer.h b/include/kvm/arm_arch_timer.h
> index b717ed9..5c970ce 100644
> --- a/include/kvm/arm_arch_timer.h
> +++ b/include/kvm/arm_arch_timer.h
> @@ -76,4 +76,5 @@ void kvm_timer_unschedule(struct kvm_vcpu *vcpu);
>  
>  void kvm_timer_vcpu_put(struct kvm_vcpu *vcpu);
>  
> +void kvm_timer_init_vhe(void);
>  #endif
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index a7fe606..6a084cd 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>
> @@ -509,3 +510,25 @@ 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)
> +{
> +	/* 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);
> +}
> diff --git a/virt/kvm/arm/hyp/timer-sr.c b/virt/kvm/arm/hyp/timer-sr.c
> index 798866a..63e28dd 100644
> --- a/virt/kvm/arm/hyp/timer-sr.c
> +++ b/virt/kvm/arm/hyp/timer-sr.c
> @@ -35,10 +35,16 @@ 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 +56,17 @@ 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);
> -- 
> 2.1.4
>