[PATCH v4 10/19] KVM: arm/arm64: Do not use kern_hyp_va() with kvm_vgic_global_state

[Marc Zyngier]

On 16/02/18 09:05, Christoffer Dall wrote:
> On Thu, Feb 15, 2018 at 01:22:56PM +0000, Marc Zyngier wrote:
>> On 15/01/18 15:36, Christoffer Dall wrote:
>>> On Thu, Jan 04, 2018 at 06:43:25PM +0000, Marc Zyngier wrote:
>>>> kvm_vgic_global_state is part of the read-only section, and is
>>>> usually accessed using a PC-relative address generation (adrp + add).
>>>>
>>>> It is thus useless to use kern_hyp_va() on it, and actively problematic
>>>> if kern_hyp_va() becomes non-idempotent. On the other hand, there is
>>>> no way that the compiler is going to guarantee that such access is
>>>> always be PC relative.
>>>
>>> nit: is always be
>>>
>>>>
>>>> So let's bite the bullet and provide our own accessor.
>>>>
>>>> Signed-off-by: Marc Zyngier <marc.zyngier at arm.com>
>>>> ---
>>>>  arch/arm/include/asm/kvm_hyp.h   | 6 ++++++
>>>>  arch/arm64/include/asm/kvm_hyp.h | 9 +++++++++
>>>>  virt/kvm/arm/hyp/vgic-v2-sr.c    | 4 ++--
>>>>  3 files changed, 17 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/arch/arm/include/asm/kvm_hyp.h b/arch/arm/include/asm/kvm_hyp.h
>>>> index ab20ffa8b9e7..1d42d0aa2feb 100644
>>>> --- a/arch/arm/include/asm/kvm_hyp.h
>>>> +++ b/arch/arm/include/asm/kvm_hyp.h
>>>> @@ -26,6 +26,12 @@
>>>>  
>>>>  #define __hyp_text __section(.hyp.text) notrace
>>>>  
>>>> +#define hyp_symbol_addr(s)						\
>>>> +	({								\
>>>> +		typeof(s) *addr = &(s);					\
>>>> +		addr;							\
>>>> +	})
>>>> +
>>>>  #define __ACCESS_VFP(CRn)			\
>>>>  	"mrc", "mcr", __stringify(p10, 7, %0, CRn, cr0, 0), u32
>>>>  
>>>> diff --git a/arch/arm64/include/asm/kvm_hyp.h b/arch/arm64/include/asm/kvm_hyp.h
>>>> index 08d3bb66c8b7..a2d98c539023 100644
>>>> --- a/arch/arm64/include/asm/kvm_hyp.h
>>>> +++ b/arch/arm64/include/asm/kvm_hyp.h
>>>> @@ -25,6 +25,15 @@
>>>>  
>>>>  #define __hyp_text __section(.hyp.text) notrace
>>>>  
>>>> +#define hyp_symbol_addr(s)						\
>>>> +	({								\
>>>> +		typeof(s) *addr;					\
>>>> +		asm volatile("adrp	%0, %1\n"			\
>>>> +			     "add	%0, %0, :lo12:%1\n"		\
>>>> +			     : "=r" (addr) : "S" (&s));			\
>>>
>>> Can't we use adr_l here?
>>
>> Unfortunately not. All the asm/assembler.h macros are unavailable to
>> inline assembly. We could start introducing equivalent macros for that
>> purpose, but that's starting to be outside of the scope of this series.
>>
> 
> Absolutely.  Forget I asked.
> 
>>>
>>>> +		addr;							\
>>>> +	})
>>>> +
>>>
>>> I don't fully appreciate the semantics of this macro going by its name
>>> only.  My understanding is that if you want to resolve a symbol to an
>>> address which is mapped in hyp, then use this.  Is this correct?
>>
>> The goal of this macro is to return a symbol's address based on a
>> PC-relative computation, as opposed to a loading the VA from a constant
>> pool or something similar. This works well for HYP, as an absolute VA is
>> guaranteed to be wrong.
>>
>>>
>>> If so, can we add a small comment (because I can't come up with a better
>>> name).
>>
>> I'll add the above if that works for you.
>>
> 
> Yes it does.  The only thing that remains a bit unclear is what the
> difference between this and kern_hyp_va is, and when you'd choose to use
> one over the other.  Perhaps we need a single place which documents our
> primitives and tells us what to use when.  At least, I'm for sure not
> going to be able to figure this out later on.

Let me try to explain that:

The two primitives work on different "objects". kern_hyp_va() works on
an address. If what you have is a pointer, then kern_hyp_va is your
friend. On the contrary, if what you have is a symbol instead of the
address of that object (and thus not something we obtain by reading a
variable), then hyp_symbol_addr is probably what you need.

Of course, a symbol can also be a variable, which makes things a bit
harder. The asm constraint are such as compilation will break if you try
to treat a local variable as a symbol (the 'S' constraint is defined as
"An absolute symbolic address or a label reference", and the '&s' makes
it pretty hard to fool).

I've tried to make it make it foolproof, but who knows... ;-)

Hope this helps,

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