linux-kernel - Re: [PATCH 08/14] KVM: arm64: Add support for tagging shared pages in page-table

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Message-ID: <CA+EHjTwmmV6EooG+Ykbso3G6nkjq=sbRzXH3vetazzPF5mO02g@mail.gmail.com>
Date:   Tue, 20 Jul 2021 14:48:09 +0100
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 08/14] KVM: arm64: Add support for tagging shared pages in page-table

Hi Quentin,

On Mon, Jul 19, 2021 at 11:47 AM Quentin Perret <qperret@...gle.com> wrote:
>
> The hypervisor will soon be in charge of tracking ownership of all
> memory pages in the system. The current page-tracking infrastructure at
> EL2 only allows binary states: a page is either owned or not by an
> entity. But a number of use-cases will require more complex states for
> pages that are shared between two entities (host, hypervisor, or guests).
>
> In preparation for supporting these use-cases, introduce in the KVM
> page-table library some infrastructure allowing to tag shared pages
> using ignored bits (a.k.a. software bits) in PTEs.
>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/include/asm/kvm_pgtable.h |  5 +++++
>  arch/arm64/kvm/hyp/pgtable.c         | 25 +++++++++++++++++++++++++
>  2 files changed, 30 insertions(+)
>
> diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> index dd72653314c7..f6d3d5c8910d 100644
> --- a/arch/arm64/include/asm/kvm_pgtable.h
> +++ b/arch/arm64/include/asm/kvm_pgtable.h
> @@ -81,6 +81,8 @@ enum kvm_pgtable_stage2_flags {
>   * @KVM_PGTABLE_PROT_W:                Write permission.
>   * @KVM_PGTABLE_PROT_R:                Read permission.
>   * @KVM_PGTABLE_PROT_DEVICE:   Device attributes.
> + * @KVM_PGTABLE_STATE_SHARED:  Page shared with another entity.
> + * @KVM_PGTABLE_STATE_BORROWED:        Page borrowed from another entity.
>   */
>  enum kvm_pgtable_prot {
>         KVM_PGTABLE_PROT_X                      = BIT(0),
> @@ -88,6 +90,9 @@ enum kvm_pgtable_prot {
>         KVM_PGTABLE_PROT_R                      = BIT(2),
>
>         KVM_PGTABLE_PROT_DEVICE                 = BIT(3),
> +
> +       KVM_PGTABLE_STATE_SHARED                = BIT(4),
> +       KVM_PGTABLE_STATE_BORROWED              = BIT(5),
>  };
>
>  #define KVM_PGTABLE_PROT_RW    (KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W)
> diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> index 5bdbe7a31551..51598b79dafc 100644
> --- a/arch/arm64/kvm/hyp/pgtable.c
> +++ b/arch/arm64/kvm/hyp/pgtable.c
> @@ -211,6 +211,29 @@ static kvm_pte_t kvm_init_invalid_leaf_owner(u8 owner_id)
>         return FIELD_PREP(KVM_INVALID_PTE_OWNER_MASK, owner_id);
>  }
>
> +static kvm_pte_t pte_ignored_bit_prot(enum kvm_pgtable_prot prot)
> +{
> +       kvm_pte_t ignored_bits = 0;
> +
> +       /*
> +        * Ignored bits 0 and 1 are reserved to track the memory ownership
> +        * state of each page:
> +        *   00: The page is owned solely by the page-table owner.
> +        *   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).
> +        */
> +       if (prot & KVM_PGTABLE_STATE_SHARED) {
> +               if (prot & KVM_PGTABLE_STATE_BORROWED)
> +                       ignored_bits |= BIT(1);
> +               else
> +                       ignored_bits |= BIT(0);
> +       }

This might tie in to Marc's comments for using enums, but
consolidating the translation between prot and ignored/software bits
in one place would be good: thinking about patch 10 as well, where you
get the prot from the ignored bits. Even though you have documented
it, I'm finding the part where a field can be borrowed and shared as
opposed to being only shared not very intuitive, and I need to reread
the comment here to remember the difference while going through the
code.

You also mention lending as potentially reserved for the future, but I
think that lending is the other side of borrowing (depends on who's
doing the giving/taking). I wonder if in this case it would be clearer
to describe it in terms of whether it's exclusively owned vs owned but
shared (for the owner), and just shared for the sharer...

Thanks,
/fuad


> +       return FIELD_PREP(KVM_PTE_LEAF_ATTR_IGNORED, ignored_bits);
> +}
> +
>  static int kvm_pgtable_visitor_cb(struct kvm_pgtable_walk_data *data, u64 addr,
>                                   u32 level, kvm_pte_t *ptep,
>                                   enum kvm_pgtable_walk_flags flag)
> @@ -357,6 +380,7 @@ static int hyp_set_prot_attr(enum kvm_pgtable_prot prot, kvm_pte_t *ptep)
>         attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_AP, ap);
>         attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S1_SH, sh);
>         attr |= KVM_PTE_LEAF_ATTR_LO_S1_AF;
> +       attr |= pte_ignored_bit_prot(prot);
>         *ptep = attr;
>
>         return 0;
> @@ -558,6 +582,7 @@ static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot p
>
>         attr |= FIELD_PREP(KVM_PTE_LEAF_ATTR_LO_S2_SH, sh);
>         attr |= KVM_PTE_LEAF_ATTR_LO_S2_AF;
> +       attr |= pte_ignored_bit_prot(prot);
>         *ptep = attr;
>
>         return 0;
> --
> 2.32.0.402.g57bb445576-goog
>