Re: [PATCH] arm64: fix documentation on kernel pages mappings to HYP VA

[Yury Norov]

On Wed, Sep 27, 2017 at 10:13:33AM +0100, Will Deacon wrote:
> On Wed, Sep 27, 2017 at 09:31:41AM +0100, Marc Zyngier wrote:
> > On Tue, Sep 26 2017 at  9:45:42 pm BST, Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx> wrote:
> > > On Wed, Sep 13, 2017 at 09:08:30PM +0300, Yury Norov wrote:
> > >> The Documentation/arm64/memory.txt says:
> > >> When using KVM, the hypervisor maps kernel pages in EL2, at a fixed
> > >> offset from the kernel VA (top 24bits of the kernel VA set to zero):
> > >> 
> > >> In fact, kernel addresses are transleted to HYP with kern_hyp_va macro,
> > >> which has more options, and none of them assumes clearing of top 24bits
> > >> of the kernel VA.
> > >> 
> > >> Signed-off-by: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx>
> > >> ---
> > >>  Documentation/arm64/memory.txt | 15 +++++++++------
> > >>  1 file changed, 9 insertions(+), 6 deletions(-)
> > >> 
> > >> diff --git a/Documentation/arm64/memory.txt b/Documentation/arm64/memory.txt
> > >> index d7273a5f6456..c39895d7e3a2 100644
> > >> --- a/Documentation/arm64/memory.txt
> > >> +++ b/Documentation/arm64/memory.txt
> > >> @@ -86,9 +86,12 @@ Translation table lookup with 64KB pages:
> > >>   +-------------------------------------------------> [63] TTBR0/1
> > >>  
> > >>  
> > >> -When using KVM, the hypervisor maps kernel pages in EL2, at a fixed
> > >> -offset from the kernel VA (top 24bits of the kernel VA set to zero):
> > >> -
> > >> -Start			End			Size		Use
> > >> ------------------------------------------------------------------------
> > >> -0000004000000000	0000007fffffffff	 256GB		kernel objects mapped in HYP
> > >> +When using KVM without Virtualization Host Extensions, the hypervisor maps
> > >> +kernel pages in EL2, at a fixed offset from the kernel VA. Namely, top 16
> > >> +or 25 bits of the kernel VA set to zero depending on ARM64_VA_BITS_48 or
> > >> +ARM64_VA_BITS_39 config option selected; or top 17 or 26 bits of the kernel
> > >> +VA set to zero if CPU has Reduced HYP mapping offset capability. See
> > >> +kern_hyp_va macro.
> > 
> > What is this "Reduced HYP mapping offset capability"?

This is the description of ARM64_HYP_OFFSET_LOW capability in
arch/arm64/kernel/cpufeature.c

> > You're missing the point that the location of the EL2 mapping is
> > conditioned by the location of the identity mapping that is used to
> > bring up / tear down KVM. You have to express the VA transformation in
> > terms of both VA_BITS (and there is more cases than just 39 or 48 bits)
> > *and* the idmap address, not to mention the case where KVM's VA_BITS is
> > larger than the rest of the kernel. See the extensive blurb in
> > kvm_mmu.h.
> > 
> > >> +
> > >> +When using KVM with Virtualization Host Extensions, no additional mappings
> > >> +created as host kernel already operates in EL2.
> > 
> > This bit is fine.
> 
> FWIW, I was going to queue a simplified version along the lines of the patch
> below.
> 
> Will

Thanks.

Yury

> --->8
> 
> commit dbf7393b7738a0ba0284551e7b6e014cfb100661
> Author: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx>
> Date:   Wed Sep 13 21:08:30 2017 +0300
> 
>     arm64: fix documentation on kernel pages mappings to HYP VA
>     
>     The Documentation/arm64/memory.txt says:
>     When using KVM, the hypervisor maps kernel pages in EL2, at a fixed
>     offset from the kernel VA (top 24bits of the kernel VA set to zero):
>     
>     In fact, kernel addresses are transleted to HYP with kern_hyp_va macro,
>     which has more options, and none of them assumes clearing of top 24bits
>     of the kernel VA.
>     
>     Signed-off-by: Yury Norov <ynorov@xxxxxxxxxxxxxxxxxx>
>     [will: removed gory details]
>     Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
> 
> diff --git a/Documentation/arm64/memory.txt b/Documentation/arm64/memory.txt
> index d7273a5f6456..ea9ee39784a2 100644
> --- a/Documentation/arm64/memory.txt
> +++ b/Documentation/arm64/memory.txt
> @@ -86,9 +86,9 @@ Translation table lookup with 64KB pages:
>   +-------------------------------------------------> [63] TTBR0/1
>  
>  
> -When using KVM, the hypervisor maps kernel pages in EL2, at a fixed
> -offset from the kernel VA (top 24bits of the kernel VA set to zero):
> +When using KVM without the Virtualization Host Extensions, the hypervisor
> +maps kernel pages in EL2 at a fixed offset from the kernel VA. See the
> +kern_hyp_va macro for more details.
>  
> -Start			End			Size		Use
> ------------------------------------------------------------------------
> -0000004000000000	0000007fffffffff	 256GB		kernel objects mapped in HYP
> +When using KVM with the Virtualization Host Extensions, no additional
> +mappings are created, since the host kernel runs directly in EL2.