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

Christoffer Dall
2016-06-28T12:03:42Z

On Mon, Jun 27, 2016 at 03:20:23PM +0100, Marc Zyngier wrote:
> On 27/06/16 14:47, Christoffer Dall wrote:
> > 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?
>
> A kernel address always has the top bits set. That's a given, and a
> property of the architecture (bits [63:VA_BITS] are set to one. See
> D4.2.1 and the definition of a Virtual Address (top VA subrange).
>

This part I understood, but I somehow had the impression that
HYP_PAGE_OFFSET_MASK could mask off more than (63 - VA_BITS + 1) bits,
but looking at the definition of HYP_PAGE_OFFSET_MASK it clearly cannot.

What can I say, I probably shouldn't have looked at code yesterday.

> >
> >> 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?
>
> That's because kern_hyp_va used to be reserved to the assembly code, and
> KERN_TO_HYP used in C code. We could (and probably should) unify them.
>
If we can, that would be good.

Thanks,
-Christoffer