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Date:   Fri, 15 Oct 2021 16:11:49 +0100
From:   Andrew Walbran <qwandor@...gle.com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Fuad Tabba <tabba@...gle.com>,
        David Brazdil <dbrazdil@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org,
        Android Kernel Team <kernel-team@...roid.com>
Subject: Re: [PATCH 01/16] KVM: arm64: Introduce do_share() helper for memory
 sharing between components

On Wed, 13 Oct 2021 at 16:58, 'Quentin Perret' via kernel-team
<kernel-team@...roid.com> wrote:
>
> From: Will Deacon <will@...nel.org>
>
> In preparation for extending memory sharing to include the guest as well
> as the hypervisor and the host, introduce a high-level do_share() helper
> which allows memory to be shared between these components without
> duplication of validity checks.
>
> Signed-off-by: Will Deacon <will@...nel.org>
> Signed-off-by: Quentin Perret <qperret@...gle.com>
> ---
>  arch/arm64/kvm/hyp/include/nvhe/mem_protect.h |   5 +
>  arch/arm64/kvm/hyp/nvhe/mem_protect.c         | 315 ++++++++++++++++++
>  2 files changed, 320 insertions(+)
>
> diff --git a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> index b58c910babaf..56445586c755 100644
> --- a/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> +++ b/arch/arm64/kvm/hyp/include/nvhe/mem_protect.h
> @@ -24,6 +24,11 @@ enum pkvm_page_state {
>         PKVM_PAGE_OWNED                 = 0ULL,
>         PKVM_PAGE_SHARED_OWNED          = KVM_PGTABLE_PROT_SW0,
>         PKVM_PAGE_SHARED_BORROWED       = KVM_PGTABLE_PROT_SW1,
> +       __PKVM_PAGE_RESERVED            = KVM_PGTABLE_PROT_SW0 |
> +                                         KVM_PGTABLE_PROT_SW1,
> +
> +       /* Meta-states which aren't encoded directly in the PTE's SW bits */
> +       PKVM_NOPAGE,
>  };
>
>  #define PKVM_PAGE_STATE_PROT_MASK      (KVM_PGTABLE_PROT_SW0 | KVM_PGTABLE_PROT_SW1)
> diff --git a/arch/arm64/kvm/hyp/nvhe/mem_protect.c b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> index bacd493a4eac..53e503501044 100644
> --- a/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> +++ b/arch/arm64/kvm/hyp/nvhe/mem_protect.c
> @@ -443,3 +443,318 @@ void handle_host_mem_abort(struct kvm_cpu_context *host_ctxt)
>         ret = host_stage2_idmap(addr);
>         BUG_ON(ret && ret != -EAGAIN);
>  }
> +
> +/* This corresponds to locking order */
> +enum pkvm_component_id {
> +       PKVM_ID_HOST,
> +       PKVM_ID_HYP,
> +};
> +
> +struct pkvm_mem_transition {
> +       u64                             nr_pages;
> +
> +       struct {
> +               enum pkvm_component_id  id;
> +               u64                     addr;
Is this the physical address or the IPA of the initiator? It would be
good to have a comment explaining.

> +
> +               union {
> +                       struct {
> +                               u64     completer_addr;
> +                       } host;
> +               };
> +       } initiator;
> +
> +       struct {
> +               enum pkvm_component_id  id;
> +       } completer;
> +};
> +
> +struct pkvm_mem_share {
> +       struct pkvm_mem_transition      tx;
> +       enum kvm_pgtable_prot           prot;
> +};
> +
> +struct pkvm_page_req {
> +       struct {
> +               enum pkvm_page_state    state;
> +               u64                     addr;
> +       } initiator;
> +
> +       struct {
> +               u64                     addr;
> +       } completer;
> +
> +       phys_addr_t                     phys;
> +};
> +
> +struct pkvm_page_share_ack {
> +       struct {
> +               enum pkvm_page_state    state;
> +               phys_addr_t             phys;
> +               enum kvm_pgtable_prot   prot;
> +       } completer;
> +};
> +
> +static void host_lock_component(void)
> +{
> +       hyp_spin_lock(&host_kvm.lock);
> +}
> +
> +static void host_unlock_component(void)
> +{
> +       hyp_spin_unlock(&host_kvm.lock);
> +}
> +
> +static void hyp_lock_component(void)
> +{
> +       hyp_spin_lock(&pkvm_pgd_lock);
> +}
> +
> +static void hyp_unlock_component(void)
> +{
> +       hyp_spin_unlock(&pkvm_pgd_lock);
> +}
> +
> +static int host_request_share(struct pkvm_page_req *req,
> +                             struct pkvm_mem_transition *tx,
> +                             u64 idx)
> +{
> +       u64 offset = idx * PAGE_SIZE;
> +       enum kvm_pgtable_prot prot;
> +       u64 host_addr;
> +       kvm_pte_t pte;
> +       int err;
> +
> +       hyp_assert_lock_held(&host_kvm.lock);
> +
> +       host_addr = tx->initiator.addr + offset;
> +       err = kvm_pgtable_get_leaf(&host_kvm.pgt, host_addr, &pte, NULL);
> +       if (err)
> +               return err;
> +
> +       if (!kvm_pte_valid(pte) && pte)
> +               return -EPERM;
> +
> +       prot = kvm_pgtable_stage2_pte_prot(pte);
> +       *req = (struct pkvm_page_req) {
> +               .initiator      = {
> +                       .state  = pkvm_getstate(prot),
> +                       .addr   = host_addr,
> +               },
> +               .completer      = {
> +                       .addr   = tx->initiator.host.completer_addr + offset,
> +               },
> +               .phys           = host_addr,
> +       };
> +
> +       return 0;
> +}
> +
> +/*
> + * Populate the page-sharing request (@req) based on the share transition
> + * information from the initiator and its current page state.
> + */
> +static int request_share(struct pkvm_page_req *req,
> +                        struct pkvm_mem_share *share,
> +                        u64 idx)
> +{
> +       struct pkvm_mem_transition *tx = &share->tx;
> +
> +       switch (tx->initiator.id) {
> +       case PKVM_ID_HOST:
> +               return host_request_share(req, tx, idx);
> +       default:
> +               return -EINVAL;
> +       }
> +}
> +
> +static int hyp_ack_share(struct pkvm_page_share_ack *ack,
> +                        struct pkvm_page_req *req,
> +                        enum kvm_pgtable_prot perms)
> +{
> +       enum pkvm_page_state state = PKVM_NOPAGE;
> +       enum kvm_pgtable_prot prot = 0;
> +       phys_addr_t phys = 0;
> +       kvm_pte_t pte;
> +       u64 hyp_addr;
> +       int err;
> +
> +       hyp_assert_lock_held(&pkvm_pgd_lock);
> +
> +       if (perms != PAGE_HYP)
> +               return -EPERM;
> +
> +       hyp_addr = req->completer.addr;
> +       err = kvm_pgtable_get_leaf(&pkvm_pgtable, hyp_addr, &pte, NULL);
> +       if (err)
> +               return err;
> +
> +       if (kvm_pte_valid(pte)) {
> +               state   = pkvm_getstate(kvm_pgtable_hyp_pte_prot(pte));
> +               phys    = kvm_pte_to_phys(pte);
> +               prot    = kvm_pgtable_hyp_pte_prot(pte) & KVM_PGTABLE_PROT_RWX;
> +       }
> +
> +       *ack = (struct pkvm_page_share_ack) {
> +               .completer      = {
> +                       .state  = state,
> +                       .phys   = phys,
> +                       .prot   = prot,
> +               },
> +       };
> +
> +       return 0;
> +}
> +
> +/*
> + * Populate the page-sharing acknowledgment (@ack) based on the sharing request
> + * from the initiator and the current page state in the completer.
> + */
> +static int ack_share(struct pkvm_page_share_ack *ack,
> +                    struct pkvm_page_req *req,
> +                    struct pkvm_mem_share *share)
> +{
> +       struct pkvm_mem_transition *tx = &share->tx;
> +
> +       switch (tx->completer.id) {
> +       case PKVM_ID_HYP:
> +               return hyp_ack_share(ack, req, share->prot);
> +       default:
> +               return -EINVAL;
> +       }
> +}
> +
> +/*
> + * Check that the page states in the initiator and the completer are compatible
> + * for the requested page-sharing operation to go ahead.
> + */
> +static int check_share(struct pkvm_page_req *req,
> +                      struct pkvm_page_share_ack *ack,
> +                      struct pkvm_mem_share *share)
> +{
> +       if (!addr_is_memory(req->phys))
> +               return -EINVAL;
> +
> +       if (req->initiator.state == PKVM_PAGE_OWNED &&
> +           ack->completer.state == PKVM_NOPAGE) {
> +               return 0;
> +       }
> +
> +       if (req->initiator.state != PKVM_PAGE_SHARED_OWNED)
> +               return -EPERM;
> +
> +       if (ack->completer.state != PKVM_PAGE_SHARED_BORROWED)
> +               return -EPERM;
> +
> +       if (ack->completer.phys != req->phys)
> +               return -EPERM;
> +
> +       if (ack->completer.prot != share->prot)
> +               return -EPERM;
I guess this is the workaround you mentioned for the fact that the
host can share the same page twice? It might be worth adding a comment
to explain that that's what's going on.

> +
> +       return 0;
> +}
> +
> +static int host_initiate_share(struct pkvm_page_req *req)
> +{
> +       enum kvm_pgtable_prot prot;
> +
> +       prot = pkvm_mkstate(PKVM_HOST_MEM_PROT, PKVM_PAGE_SHARED_OWNED);
> +       return host_stage2_idmap_locked(req->initiator.addr, PAGE_SIZE, prot);
> +}
> +
> +/* Update the initiator's page-table for the page-sharing request */
> +static int initiate_share(struct pkvm_page_req *req,
> +                         struct pkvm_mem_share *share)
> +{
> +       struct pkvm_mem_transition *tx = &share->tx;
> +
> +       switch (tx->initiator.id) {
> +       case PKVM_ID_HOST:
> +               return host_initiate_share(req);
> +       default:
> +               return -EINVAL;
> +       }
> +}
> +
> +static int hyp_complete_share(struct pkvm_page_req *req,
> +                             enum kvm_pgtable_prot perms)
> +{
> +       void *start = (void *)req->completer.addr, *end = start + PAGE_SIZE;
> +       enum kvm_pgtable_prot prot;
> +
> +       prot = pkvm_mkstate(perms, PKVM_PAGE_SHARED_BORROWED);
> +       return pkvm_create_mappings_locked(start, end, prot);
> +}
> +
> +/* Update the completer's page-table for the page-sharing request */
> +static int complete_share(struct pkvm_page_req *req,
> +                         struct pkvm_mem_share *share)
> +{
> +       struct pkvm_mem_transition *tx = &share->tx;
> +
> +       switch (tx->completer.id) {
> +       case PKVM_ID_HYP:
> +               return hyp_complete_share(req, share->prot);
> +       default:
> +               return -EINVAL;
> +       }
> +}
> +
> +/*
> + * do_share():
> + *
> + * The page owner grants access to another component with a given set
> + * of permissions.
> + *
> + * Initiator: OWNED    => SHARED_OWNED
> + * Completer: NOPAGE   => SHARED_BORROWED
> + *
> + * Note that we permit the same share operation to be repeated from the
> + * host to the hypervisor, as this removes the need for excessive
> + * book-keeping of shared KVM data structures at EL1.
> + */
> +static int do_share(struct pkvm_mem_share *share)
> +{
> +       struct pkvm_page_req req;
> +       int ret = 0;
> +       u64 idx;
> +
> +       for (idx = 0; idx < share->tx.nr_pages; ++idx) {
> +               struct pkvm_page_share_ack ack;
> +
> +               ret = request_share(&req, share, idx);
> +               if (ret)
> +                       goto out;
> +
> +               ret = ack_share(&ack, &req, share);
> +               if (ret)
> +                       goto out;
> +
> +               ret = check_share(&req, &ack, share);
> +               if (ret)
> +                       goto out;
> +       }
> +
> +       for (idx = 0; idx < share->tx.nr_pages; ++idx) {
> +               ret = request_share(&req, share, idx);
> +               if (ret)
> +                       break;
> +
> +               /* Allow double-sharing by skipping over the page */
> +               if (req.initiator.state == PKVM_PAGE_SHARED_OWNED)
> +                       continue;
> +
> +               ret = initiate_share(&req, share);
> +               if (ret)
> +                       break;
> +
> +               ret = complete_share(&req, share);
> +               if (ret)
> +                       break;
> +       }
> +
> +       WARN_ON(ret);
> +out:
> +       return ret;
> +}
> --
> 2.33.0.882.g93a45727a2-goog
>
> --
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>

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