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

Will Deacon
2017-09-27T09:13:22Z

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@caviumnetworks.com> 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@caviumnetworks.com>
> >> ---
> >> 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"?
>
> 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

--->8

commit dbf7393b7738a0ba0284551e7b6e014cfb100661
Author: Yury Norov <ynorov@caviumnetworks.com>
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@caviumnetworks.com>
[will: removed gory details]
Signed-off-by: Will Deacon <will.deacon@arm.com>

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.