Re: [PATCH v18 060/121] KVM: TDX: TDP MMU TDX support

[Binbin Wu]

On 1/23/2024 7:53 AM, isaku.yamahata@xxxxxxxxx wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
>
> 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@xxxxxxxxx>
>
> ---
> 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.
> ---
>   arch/x86/kvm/mmu/spte.c    |   3 +-
>   arch/x86/kvm/vmx/main.c    |  71 +++++++-
>   arch/x86/kvm/vmx/tdx.c     | 342 +++++++++++++++++++++++++++++++++++++
>   arch/x86/kvm/vmx/tdx.h     |   2 +-
>   arch/x86/kvm/vmx/tdx_ops.h |   6 +
>   arch/x86/kvm/vmx/x86_ops.h |   6 +
>   6 files changed, 424 insertions(+), 6 deletions(-)
[...]
> +
> +/*
> + * 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.
> +	 */
> +	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 {
> +		/*
> +		 * kvm_flush_remote_tlbs() doesn't allow to return error and
> +		 * retry.
> +		 */
> +		err = tdh_mem_track(kvm_tdx->tdr_pa);
> +	} while (unlikely((err & TDX_SEAMCALL_STATUS_MASK) == TDX_OPERAND_BUSY));

Why the sequence of the code is different from the description of the 
function.
In the description, do the TDH.MEM.TRACK before IPIs.
But in the code, do TDH.MEM.TRACK after IPIs?


> +
> +	/* 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);
> +
> +}
> +
[...]