linux-kernel - Re: [PATCH] KVM: arm64: Update comment of create_hyp

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Date:   Fri, 11 Nov 2022 16:18:34 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Wei-Lin Chang <r09922117@...e.ntu.edu.tw>
Cc:     james.morse@....com, alexandru.elisei@....com,
        suzuki.poulose@....com, oliver.upton@...ux.dev,
        catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: arm64: Update comment of create_hyp_mappings

On Thu, 03 Nov 2022 05:32:28 +0000,
Wei-Lin Chang <r09922117@...e.ntu.edu.tw> wrote:
> 
> HYP_PAGE_OFFSET is removed since 4.8, and the method for generating Hyp
> VAs has evolved. Update the functional description of
> create_hyp_mappings accordingly.
> 
> Signed-off-by: Wei-Lin Chang <r09922117@...e.ntu.edu.tw>
> ---
>  arch/arm64/kvm/mmu.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index c9a13e487..a9ae4a3f9 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -424,9 +424,10 @@ void kvm_unshare_hyp(void *from, void *to)
>   * @to:		The virtual kernel end address of the range (exclusive)
>   * @prot:	The protection to be applied to this range
>   *
> - * The same virtual address as the kernel virtual address is also used
> - * in Hyp-mode mapping (modulo HYP_PAGE_OFFSET) to the same underlying
> - * physical pages.
> + * The Hyp virtual address is generated by masking the kernel VA with
> + * va_mask then inserting tag_val for the higher bits starting from
> + * tag_lsb. See kvm_compute_layout() in va_layout.c for more info.
> + * Both Hyp VA and kernel VA ranges map to the same underlying physical pages.

My problem with this comment is that neither va_mask, tag_val, nor
tag_lsb mean anything in this context. All this is purely internal to
kvm_compute_layout(), and is unnecessary here.

I'd rather you have something along the lines of:

diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 60ee3d9f01f8..6d04818a1a5b 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -460,7 +460,7 @@ void kvm_unshare_hyp(void *from, void *to)
  * @prot:	The protection to be applied to this range
  *
  * The same virtual address as the kernel virtual address is also used
- * in Hyp-mode mapping (modulo HYP_PAGE_OFFSET) to the same underlying
+ * in Hyp-mode mapping (modulo a random offset) to the same underlying
  * physical pages.
  */
 int create_hyp_mappings(void *from, void *to, enum kvm_pgtable_prot prot)

Whoever is interested in understanding the generation of the offset
can follow kern_hyp_va().

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.