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

Marc Zyngier
2016-12-06T13:09:26Z

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?

Thanks,

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