[PATCH v5 19/23] arm64: KVM: Allow far branches from vector slots to the main vectors

[marc.zyngier@arm.com (Marc Zyngier)]

Hi James,

On 12/03/18 18:27, James Morse wrote:
> Hi Marc,
> 
> On 01/03/18 15:55, Marc Zyngier wrote:
>> So far, the branch from the vector slots to the main vectors can at
>> most be 4GB from the main vectors (the reach of ADRP), and this
>> distance is known at compile time. If we were to remap the slots
>> to an unrelated VA, things would break badly.
>>
>> A way to achieve VA independence would be to load the absolute
>> address of the vectors (__kvm_hyp_vector), either using a constant
>> pool or a series of movs, followed by an indirect branch.
>>
>> This patches implements the latter solution, using another instance
>> of a patching callback.
> 
>> diff --git a/arch/arm64/kernel/bpi.S b/arch/arm64/kernel/bpi.S
>> index e000cb390618..e8d997788ad0 100644
>> --- a/arch/arm64/kernel/bpi.S
>> +++ b/arch/arm64/kernel/bpi.S
>> @@ -19,6 +19,9 @@
>>  #include <linux/linkage.h>
>>  #include <linux/arm-smccc.h>
>>  
>> +#include <asm/alternative.h>
>> +#include <asm/kvm_mmu.h>
>> +
>>  .macro hyp_ventry offset
>>  	.align 7
>>  	.rept 29
>> @@ -64,9 +67,15 @@ ENTRY(__bp_harden_hyp_vecs_start)
>>  	.endr
>>  
>>  __kvm_enter_vectors:
>> +alternative_cb	kvm_patch_vector_branch
>> +	movz	x1, #0
>> +	movk	x1, #0, lsl #16
>> +	movk	x1, #0, lsl #32
>> +	movk	x1, #0, lsl #48
>> +alternative_cb_end
>>  
>> -	adr_l	x1, __kvm_hyp_vector
>>  	add	x0, x1, x0
>> +	kern_hyp_va x0
> 
> Can't you patch the kern_hyp_va address into the movk block directly?
> Obviously you can't call kern_hyp_va, but you could generate the layout and have
> some slow __kern_hyp_va() to generate the value. This would avoid generating a
> value, to then throw half of it away and patch something else in.

Hmmm. That's pretty twisted. Actually, this is utterly terrifying. And
thus absolutely mandatory. Irk. I really like it.

> Does this code run for VHE systems too? (it not, is the x<<48 movk needed?)

This is indeed VHE only, and if/when I adopt your suggestion, we'll be
able to drop the last movk, effectively getting rid of 6 instructions on
the exception path. Awesome!

Thanks,

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