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

[Marc Zyngier]

On 28/11/16 19:42, Christoffer Dall wrote:
> On Mon, Nov 28, 2016 at 06:39:04PM +0000, Marc Zyngier wrote:
>> On 28/11/16 17:43, Marc Zyngier wrote:
>>> Hi Jintack,
>>>
>>> On 28/11/16 16:46, Jintack Lim wrote:
>>>> 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.  Current code is
>>>> unintentionally setting wrong bits to CNTHCTL_EL2 with E2H set, which
>>>> may allow guest OS to access physical timer. So, fix it.
>>>>
>>>> Signed-off-by: Jintack Lim <jintack at cs.columbia.edu>
>>>> ---
>>>>  arch/arm/include/asm/kvm_timer.h     | 33 +++++++++++++++++++
>>>>  arch/arm64/include/asm/kvm_timer.h   | 62 ++++++++++++++++++++++++++++++++++++
>>>>  include/clocksource/arm_arch_timer.h |  6 ++--
>>>>  virt/kvm/arm/hyp/timer-sr.c          |  8 ++---
>>>>  4 files changed, 103 insertions(+), 6 deletions(-)
>>>>  create mode 100644 arch/arm/include/asm/kvm_timer.h
>>>>  create mode 100644 arch/arm64/include/asm/kvm_timer.h
>>>>
>>
>> [...]
>>
>>> We could make it nicer (read "faster") by introducing a
>>> hyp_alternate_select construct that only returns a value instead
>>> of calling a function. I remember writing something like that
>>> at some point, and dropping it...
>>
>> So here's what this could look like (warning, wacky code ahead,
>> though I fixed a stupid bug that was present in the previous patch).
>> The generated code is quite nice (no branch, only an extra mov
>> instruction on the default path)... Of course, completely untested!
> 
> Isn't this all about determining which bitmask to use, statically, once,
> after the system has booted?
> 
> How about a good old fashioned static variable, or global struct like
> the global one we use for the VGIC, which sets the proper mit mask
> during kvm init, and the world-switch code just uses a variable?

We could indeed do that (I've been carried away with my tendency for
weird and wonderful hacks).

But as Jintack mentioned, there is a much better approach, which is to
do nothing at all on the VHE path (we can set the permission bits once
and for all). cntvoff_el2 also falls into the same category of things we
should be able to only restore and not bother resetting (as it doesn't
affect the EL2 virtual counter).

Thoughts?

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