linux-kernel - Re: [PATCH v2 07/21] arm64: KVM: VHE: Patch out kern_hyp

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Date:	Mon, 1 Feb 2016 16:40:52 +0100
From:	Christoffer Dall <christoffer.dall@...aro.org>
To:	Marc Zyngier <marc.zyngier@....com>
Cc:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, kvmarm@...ts.cs.columbia.edu
Subject: Re: [PATCH v2 07/21] arm64: KVM: VHE: Patch out kern_hyp_va

On Mon, Feb 01, 2016 at 01:38:57PM +0000, Marc Zyngier wrote:
> On 01/02/16 13:20, Christoffer Dall wrote:
> > 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@....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?
> 
> We commonly pass both pointers and unsigned long to this helper. Do you
> really want a separate helper for each type instead of one that does it all?
> 

no, what I was suggesting was to always take a specific type and change
the callers that don't match to use a specific cast so you give the
compiler a chance to scream at you when writing new code.

But I don't feel strongly about it.

Thanks,
-Christoffer