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

Christoffer Dall
2016-12-06T14:27:00Z

On Tue, Dec 06, 2016 at 01:09:26PM +0000, Marc Zyngier wrote:
> On 06/12/16 12:16, Christoffer Dall wrote:
> > On Tue, Dec 06, 2016 at 01:12:21PM +0100, Christoffer Dall wrote:
> >> On Tue, Dec 06, 2016 at 11:17:40AM +0000, Marc Zyngier wrote:
> >>> 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...).
> >>>
> >>
> >> Isn't this code called from a file shared between 32-bit arm and arm64?
> >> Does the cpus_have_const_cap work on ARM64?
> >
> > Duh, I meant on 32-bit arm of course.
>
> Well, this is a pure 64bit file - 32bit has the canonical implementation
> that always returns false. So I can't really see how this can ever break
> 32bit. Unless my lack of sleep is really showing, and I'm missing
> something terribly obvious?
>
No, I'm being an idiot, too many things at once and a lack of coffee.

-Christoffer