linux-kernel - Re: [PATCH v3 12/21] KVM: arm64: Add helpers to tag shared pages in SW bits

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Date:   Mon, 2 Aug 2021 12:30:25 +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 12/21] KVM: arm64: Add helpers to tag shared pages in
 SW bits

Hi Quentin,


On Thu, Jul 29, 2021 at 3:28 PM Quentin Perret <qperret@...gle.com> wrote:
>
> We will soon start annotating shared pages in page-tables in nVHE
> protected mode. Define all the states in which a page can be (owned,
> shared and owned, shared and borrowed), and provide helpers allowing to
> convert this into SW bits annotations using the matching prot
> attributes.
>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/mem_protect.h | 26 +++++++++++++++++++
>  1 file changed, 26 insertions(+)
>
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> index 9c227d87c36d..ae355bfd8c01 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> @@ -12,6 +12,32 @@
>  #include <asm/virt.h>
>  #include <nvhe/spinlock.h>
>
> +/*
> + * SW bits 0-1 are reserved to track the memory ownership state of each page:
> + *   00: The page is owned solely by the page-table owner.

nit: solely -> exclusively, because "exclusive" is the more common
term in context of shared resources

> + *   01: The page is owned by the page-table owner, but is shared
> + *       with another entity.
> + *   10: The page is shared with, but not owned by the page-table owner.
> + *   11: Reserved for future use (lending).
> + */
> +enum pkvm_page_state {
> +       PKVM_PAGE_OWNED                 = 0ULL,
> +       PKVM_PAGE_SHARED_OWNED          = KVM_PGTABLE_PROT_SW0,
> +       PKVM_PAGE_SHARED_BORROWED       = KVM_PGTABLE_PROT_SW1,
> +};
> +
> +#define PKVM_PAGE_STATE_PROT_MASK      (KVM_PGTABLE_PROT_SW0 | KVM_PGTABLE_PROT_SW1)
> +static inline enum kvm_pgtable_prot pkvm_mkstate(enum kvm_pgtable_prot prot,
> +                                                enum pkvm_page_state state)
> +{
> +       return (prot & ~PKVM_PAGE_STATE_PROT_MASK) | state;
> +}
> +
> +static inline enum pkvm_page_state pkvm_getstate(enum kvm_pgtable_prot prot)
> +{
> +       return prot & PKVM_PAGE_STATE_PROT_MASK;
> +}
> +

I think that this encoding is pretty neat and easy to follow.

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

Thanks,
/fuad

>  struct host_kvm {
>         struct kvm_arch arch;
>         struct kvm_pgtable pgt;
> --
> 2.32.0.432.gabb21c7263-goog
>