Re: [PATCH 02/15] arm64: KVM: Kill HYP_PAGE_OFFSET

[Christoffer Dall <christoffer.dall@linaro.org>]

On Tue, Jun 07, 2016 at 11:58:22AM +0100, Marc Zyngier wrote:
> HYP_PAGE_OFFSET is not massively useful. And the way we use it
> in KERN_HYP_VA is inconsistent with the equivalent operation in
> EL2, where we use a mask instead.
> 
> Let's replace the uses of HYP_PAGE_OFFSET with HYP_PAGE_OFFSET_MASK,
> and get rid of the pointless macro.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
>  arch/arm64/include/asm/kvm_hyp.h | 5 ++---
>  arch/arm64/include/asm/kvm_mmu.h | 3 +--
>  2 files changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_hyp.h b/arch/arm64/include/asm/kvm_hyp.h
> index 44eaff7..61d01a9 100644
> --- a/arch/arm64/include/asm/kvm_hyp.h
> +++ b/arch/arm64/include/asm/kvm_hyp.h
> @@ -38,11 +38,10 @@ static inline unsigned long __kern_hyp_va(unsigned long v)
>  
>  static inline unsigned long __hyp_kern_va(unsigned long v)
>  {
> -	u64 offset = PAGE_OFFSET - HYP_PAGE_OFFSET;
> -	asm volatile(ALTERNATIVE("add %0, %0, %1",
> +	asm volatile(ALTERNATIVE("orr %0, %0, %1",
>  				 "nop",
>  				 ARM64_HAS_VIRT_HOST_EXTN)
> -		     : "+r" (v) : "r" (offset));
> +		     : "+r" (v) : "i" (~HYP_PAGE_OFFSET_MASK));

for some reason this is hurting my brain.  I can't easily see that the
two implementations are equivalent.

I can see that the kernel-to-hyp masking is trivially correct, but are
we always sure that the upper bits that we mask off are always set?

>  	return v;
>  }
>  
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 00bc277..d162372 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -75,7 +75,6 @@
>   */
>  #define HYP_PAGE_OFFSET_SHIFT	VA_BITS
>  #define HYP_PAGE_OFFSET_MASK	((UL(1) << HYP_PAGE_OFFSET_SHIFT) - 1)
> -#define HYP_PAGE_OFFSET		(PAGE_OFFSET & HYP_PAGE_OFFSET_MASK)
>  
>  /*
>   * Our virtual mapping for the idmap-ed MMU-enable code. Must be
> @@ -109,7 +108,7 @@ alternative_endif
>  #include <asm/mmu_context.h>
>  #include <asm/pgtable.h>
>  
> -#define KERN_TO_HYP(kva)	((unsigned long)kva - PAGE_OFFSET + HYP_PAGE_OFFSET)
> +#define KERN_TO_HYP(kva)	((unsigned long)kva & HYP_PAGE_OFFSET_MASK)
>  

Why do we have both kern_hyp_va() and KERN_TO_HYP and how are they
related again?

Thanks,
-Christoffer