Re: [PATCH v2 07/21] arm64: KVM: VHE: Patch out kern_hyp_va

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

On Mon, Jan 25, 2016 at 03:53:41PM +0000, Marc Zyngier wrote:
> The kern_hyp_va macro is pretty meaninless with VHE, as there is
> only one mapping - the kernel one.
> 
> In order to keep the code readable and efficient, use runtime
> patching to replace the 'and' instruction used to compute the VA
> with a 'nop'.
> 
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
>  arch/arm64/include/asm/kvm_mmu.h | 11 ++++++++++-
>  arch/arm64/kvm/hyp/hyp.h         | 25 ++++++++++++++++++++++---
>  2 files changed, 32 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index d3e6d7b..62f0d14 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -23,13 +23,16 @@
>  #include <asm/cpufeature.h>
>  
>  /*
> - * As we only have the TTBR0_EL2 register, we cannot express
> + * As ARMv8.0 only has the TTBR0_EL2 register, we cannot express
>   * "negative" addresses. This makes it impossible to directly share
>   * mappings with the kernel.
>   *
>   * Instead, give the HYP mode its own VA region at a fixed offset from
>   * the kernel by just masking the top bits (which are all ones for a
>   * kernel address).
> + *
> + * ARMv8.1 (using VHE) does have a TTBR1_EL2, and doesn't use these
> + * macros (the entire kernel runs at EL2).
>   */
>  #define HYP_PAGE_OFFSET_SHIFT	VA_BITS
>  #define HYP_PAGE_OFFSET_MASK	((UL(1) << HYP_PAGE_OFFSET_SHIFT) - 1)
> @@ -56,6 +59,8 @@
>  
>  #ifdef __ASSEMBLY__
>  
> +#include <asm/alternative.h>
> +#include <asm/cpufeature.h>
>  #include <asm/kvm_arm.h>
>  
>  .macro setup_vtcr tmp1, tmp2
> @@ -84,7 +89,11 @@
>   * reg: VA to be converted.
>   */
>  .macro kern_hyp_va	reg
> +alternative_if_not ARM64_HAS_VIRT_HOST_EXTN	
>  	and	\reg, \reg, #HYP_PAGE_OFFSET_MASK
> +alternative_else
> +	nop
> +alternative_endif
>  .endm
>  
>  #else
> diff --git a/arch/arm64/kvm/hyp/hyp.h b/arch/arm64/kvm/hyp/hyp.h
> index fb27517..fc502f3 100644
> --- a/arch/arm64/kvm/hyp/hyp.h
> +++ b/arch/arm64/kvm/hyp/hyp.h
> @@ -25,9 +25,28 @@
>  
>  #define __hyp_text __section(.hyp.text) notrace
>  
> -#define kern_hyp_va(v) (typeof(v))((unsigned long)(v) & HYP_PAGE_OFFSET_MASK)
> -#define hyp_kern_va(v) (typeof(v))((unsigned long)(v) - HYP_PAGE_OFFSET \
> -						      + PAGE_OFFSET)
> +static inline unsigned long __kern_hyp_va(unsigned long v)
> +{
> +	asm volatile(ALTERNATIVE("and %0, %0, %1",
> +				 "nop",
> +				 ARM64_HAS_VIRT_HOST_EXTN)
> +		     : "+r" (v) : "i" (HYP_PAGE_OFFSET_MASK));
> +	return v;
> +}
> +
> +#define kern_hyp_va(v) (typeof(v))(__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",
> +				 "nop",
> +				 ARM64_HAS_VIRT_HOST_EXTN)
> +		     : "+r" (v) : "r" (offset));
> +	return v;
> +}
> +
> +#define hyp_kern_va(v) (typeof(v))(__hyp_kern_va((unsigned long)(v)))

why do we need this casting of values instead of just defining these
inlines and calling them directly with proper typing?

-Christoffer

>  
>  /**
>   * hyp_alternate_select - Generates patchable code sequences that are
> -- 
> 2.1.4
>