Re: [PATCH v19 070/130] KVM: TDX: TDP MMU TDX support

Binbin Wu
2024-04-02T09:13:29Z

On 2/26/2024 4:26 PM, isaku.yamahata@intel.com wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
>
> Implement hooks of TDP MMU for TDX backend. TLB flush, TLB shootdown,
> propagating the change private EPT entry to Secure EPT and freeing Secure
> EPT page. TLB flush handles both shared EPT and private EPT. It flushes
> shared EPT same as VMX. It also waits for the TDX TLB shootdown. For the
> hook to free Secure EPT page, unlinks the Secure EPT page from the Secure
> EPT so that the page can be freed to OS.
>
> Propagate the entry change to Secure EPT. The possible entry changes are
> present -> non-present(zapping) and non-present -> present(population). On
> population just link the Secure EPT page or the private guest page to the
> Secure EPT by TDX SEAMCALL. Because TDP MMU allows concurrent
> zapping/population, zapping requires synchronous TLB shoot down with the
> frozen EPT entry. It zaps the secure entry, increments TLB counter, sends
> IPI to remote vcpus to trigger TLB flush, and then unlinks the private
> guest page from the Secure EPT. For simplicity, batched zapping with
> exclude lock is handled as concurrent zapping. Although it's inefficient,
> it can be optimized in the future.
>
> For MMIO SPTE, the spte value changes as follows.
> initial value (suppress VE bit is set)
> -> Guest issues MMIO and triggers EPT violation
> -> KVM updates SPTE value to MMIO value (suppress VE bit is cleared)
> -> Guest MMIO resumes. It triggers VE exception in guest TD
> -> Guest VE handler issues TDG.VP.VMCALL<MMIO>
> -> KVM handles MMIO
> -> Guest VE handler resumes its execution after MMIO instruction
>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>
> ---
> v19:
> - Compile fix when CONFIG_HYPERV != y.
> It's due to the following patch. Catch it up.
> https://lore.kernel.org/all/20231018192325.1893896-1-seanjc@google.com/
> - Add comments on tlb shootdown to explan the sequence.
> - Use gmem_max_level callback, delete tdp_max_page_level.
>
> v18:
> - rename tdx_sept_page_aug() -> tdx_mem_page_aug()
> - checkpatch: space => tab
>
> v15 -> v16:
> - Add the handling of TD_ATTR_SEPT_VE_DISABLE case.
>
> v14 -> v15:
> - Implemented tdx_flush_tlb_current()
> - Removed unnecessary invept in tdx_flush_tlb(). It was carry over
> from the very old code base.
>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> ---
> arch/x86/kvm/mmu/spte.c | 3 +-
> arch/x86/kvm/vmx/main.c | 91 ++++++++-
> arch/x86/kvm/vmx/tdx.c | 372 +++++++++++++++++++++++++++++++++++++
> arch/x86/kvm/vmx/tdx.h | 2 +-
> arch/x86/kvm/vmx/tdx_ops.h | 6 +
> arch/x86/kvm/vmx/x86_ops.h | 13 ++
> 6 files changed, 481 insertions(+), 6 deletions(-)
>
[...]

> +static int tdx_sept_zap_private_spte(struct kvm *kvm, gfn_t gfn,
> + enum pg_level level)
> +{
> + int tdx_level = pg_level_to_tdx_sept_level(level);
> + struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> + gpa_t gpa = gfn_to_gpa(gfn) & KVM_HPAGE_MASK(level);
> + struct tdx_module_args out;
> + u64 err;
> +
> + /* This can be called when destructing guest TD after freeing HKID. */
> + if (unlikely(!is_hkid_assigned(kvm_tdx)))
> + return 0;
> +
> + /* For now large page isn't supported yet. */
> + WARN_ON_ONCE(level != PG_LEVEL_4K);
> + err = tdh_mem_range_block(kvm_tdx->tdr_pa, gpa, tdx_level, &out);
> + if (unlikely(err == TDX_ERROR_SEPT_BUSY))
> + return -EAGAIN;
> + if (KVM_BUG_ON(err, kvm)) {
> + pr_tdx_error(TDH_MEM_RANGE_BLOCK, err, &out);
> + return -EIO;
> + }
> + return 0;
> +}
> +
> +/*
> + * TLB shoot down procedure:
> + * There is a global epoch counter and each vcpu has local epoch counter.
> + * - TDH.MEM.RANGE.BLOCK(TDR. level, range) on one vcpu
> + * This blocks the subsequenct creation of TLB translation on that range.
> + * This corresponds to clear the present bit(all RXW) in EPT entry
> + * - TDH.MEM.TRACK(TDR): advances the epoch counter which is global.
> + * - IPI to remote vcpus
> + * - TDExit and re-entry with TDH.VP.ENTER on remote vcpus
> + * - On re-entry, TDX module compares the local epoch counter with the global
> + * epoch counter. If the local epoch counter is older than the global epoch
> + * counter, update the local epoch counter and flushes TLB.
> + */
> +static void tdx_track(struct kvm *kvm)
> +{
> + struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> + u64 err;
> +
> + KVM_BUG_ON(!is_hkid_assigned(kvm_tdx), kvm);
> + /* If TD isn't finalized, it's before any vcpu running. */
> + if (unlikely(!is_td_finalized(kvm_tdx)))
> + return;
> +
> + /*
> + * tdx_flush_tlb() waits for this function to issue TDH.MEM.TRACK() by
> + * the counter. The counter is used instead of bool because multiple
> + * TDH_MEM_TRACK() can be issued concurrently by multiple vcpus.

Which case will have concurrent issues of TDH_MEM_TRACK() by multiple vcpus?
For now, zapping is holding write lock.
Promotion/demotion may have concurrent issues of TDH_MEM_TRACK(), but
it's not supported yet.

> + *
> + * optimization: The TLB shoot down procedure described in The TDX
> + * specification is, TDH.MEM.TRACK(), send IPI to remote vcpus, confirm
> + * all remote vcpus exit to VMM, and execute vcpu, both local and
> + * remote. Twist the sequence to reduce IPI overhead as follows.
> + *
> + * local remote
> + * ----- ------
> + * increment tdh_mem_track
> + *
> + * request KVM_REQ_TLB_FLUSH
> + * send IPI
> + *
> + * TDEXIT to KVM due to IPI
> + *
> + * IPI handler calls tdx_flush_tlb()
> + * to process KVM_REQ_TLB_FLUSH.
> + * spin wait for tdh_mem_track == 0
> + *
> + * TDH.MEM.TRACK()
> + *
> + * decrement tdh_mem_track
> + *
> + * complete KVM_REQ_TLB_FLUSH
> + *
> + * TDH.VP.ENTER to flush tlbs TDH.VP.ENTER to flush tlbs
> + */
> + atomic_inc(&kvm_tdx->tdh_mem_track);
> + /*
> + * KVM_REQ_TLB_FLUSH waits for the empty IPI handler, ack_flush(), with
> + * KVM_REQUEST_WAIT.
> + */
> + kvm_make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH);
> +
> + do {
> + err = tdh_mem_track(kvm_tdx->tdr_pa);
> + } while (unlikely((err & TDX_SEAMCALL_STATUS_MASK) == TDX_OPERAND_BUSY));
> +
> + /* Release remote vcpu waiting for TDH.MEM.TRACK in tdx_flush_tlb(). */
> + atomic_dec(&kvm_tdx->tdh_mem_track);
> +
> + if (KVM_BUG_ON(err, kvm))
> + pr_tdx_error(TDH_MEM_TRACK, err, NULL);
> +
> +}
> +
> +static int tdx_sept_free_private_spt(struct kvm *kvm, gfn_t gfn,
> + enum pg_level level, void *private_spt)
> +{
> + struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> +
> + /*
> + * The HKID assigned to this TD was already freed and cache was
> + * already flushed. We don't have to flush again.
> + */
> + if (!is_hkid_assigned(kvm_tdx))
> + return tdx_reclaim_page(__pa(private_spt));
> +
> + /*
> + * free_private_spt() is (obviously) called when a shadow page is being
> + * zapped. KVM doesn't (yet) zap private SPs while the TD is active.
> + * Note: This function is for private shadow page. Not for private
> + * guest page. private guest page can be zapped during TD is active.
> + * shared <-> private conversion and slot move/deletion.
> + */
> + KVM_BUG_ON(is_hkid_assigned(kvm_tdx), kvm);

At this point, is_hkid_assigned(kvm_tdx) is always true.