linux-kernel - Re: [PATCH v3 08/21] KVM: arm64: Don't overwrite software bits with owner id

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Date:   Mon, 2 Aug 2021 11:38:31 +0200
From:   Fuad Tabba <tabba@...gle.com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     maz@...nel.org, james.morse@....com, alexandru.elisei@....com,
        suzuki.poulose@....com, catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org, ardb@...nel.org, qwandor@...gle.com,
        dbrazdil@...gle.com, kernel-team@...roid.com
Subject: Re: [PATCH v3 08/21] KVM: arm64: Don't overwrite software bits with
 owner id

Hi Quentin,

On Thu, Jul 29, 2021 at 3:28 PM Quentin Perret <qperret@...gle.com> wrote:
>
> We will soon start annotating page-tables with new flags to track shared
> pages and such, and we will do so in valid mappings using software bits
> in the PTEs, as provided by the architecture. However, it is possible
> that we will need to use those flags to annotate invalid mappings as
> well in the future, similar to what we do to track page ownership in the
> host stage-2.
>
> In order to facilitate the annotation of invalid mappings with such
> flags, it would be preferable to re-use the same bits as for valid
> mappings (bits [58-55]), but these are currently used for ownership
> encoding. Since we have plenty of bits left to use in invalid
> mappings, move the ownership bits further down the PTE to avoid the
> conflict.
>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/kvm/hyp/pgtable.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 59a394d82de3..1ee1168ac32d 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -46,7 +46,7 @@
>                                          KVM_PTE_LEAF_ATTR_LO_S2_S2AP_W | \
>                                          KVM_PTE_LEAF_ATTR_HI_S2_XN)
>
> -#define KVM_INVALID_PTE_OWNER_MASK     GENMASK(63, 56)
> +#define KVM_INVALID_PTE_OWNER_MASK     GENMASK(9, 2)
>  #define KVM_MAX_OWNER_ID               1

This change makes sense so that the same bits can be used to track sharing.

Reviewed-by: Fuad Tabba <tabba@...gle.com>

Thanks,
/fuad

>
>  struct kvm_pgtable_walk_data {

> --
> 2.32.0.432.gabb21c7263-goog
>