Re: [PATCH v3] KVM: arm/arm64: Access CNTHCTL_EL2 bit fields correctly

Marc Zyngier
2016-12-06T11:17:40Z

On 01/12/16 19:32, 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@cs.columbia.edu>
> ---
> v2->v3:
> - Perform the initialization including CPU hotplug case.
> - Move has_vhe() to asm/virt.h
>
> v1->v2:
> - Skip configuring cnthctl_el2 in world switch path on VHE system.
> - Write patch based on linux-next.
> ---
> arch/arm/include/asm/virt.h | 5 +++++
> arch/arm/kvm/arm.c | 3 +++
> arch/arm64/include/asm/virt.h | 10 ++++++++++
> 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, 63 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 8f92efa..13e54e8 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..b043cfd 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 +81,15 @@ static inline bool is_kernel_in_hyp_mode(void)
> return read_sysreg(CurrentEL) == CurrentEL_EL2;
> }
>
> +static inline bool has_vhe(void)
> +{
> +#ifdef CONFIG_ARM64_VHE

Is there a particular reason why this should be guarded by a #ifdef? All
the symbols should always be available, and since this is a static key,
the overhead should be really insignificant (provided that you use a
non-prehistoric compiler...).

> + if (cpus_have_const_cap(ARM64_HAS_VIRT_HOST_EXTN))
> + return true;
> +#endif
> + return false;
> +}
> +
> #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 dda39d8..2d54903 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);
> #endif
> diff --git a/virt/kvm/arm/arch_timer.c b/virt/kvm/arm/arch_timer.c
> index 17b8fa5..c7c3bfd 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)
> +{
> + /* 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);
>

Otherwise, and assuming you're OK with me fixing the above nit (no need
to resend):

Acked-by: Marc Zyngier <marc.zyngier@arm.com>

Thanks,

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