Re: [PATCH 4/7] KVM: TDX: Kick off vCPUs when SEAMCALL is busy during TD page removal

Yan Zhao
2025-01-16T08:19:27Z

On Thu, Jan 16, 2025 at 02:23:24PM +0800, Binbin Wu wrote:
>
>
>
> On 1/13/2025 10:12 AM, Yan Zhao wrote:
> [...]
> > +
> > /* TDH.PHYMEM.PAGE.RECLAIM is allowed only when destroying the TD. */
> > static int __tdx_reclaim_page(hpa_t pa)
> > {
> > @@ -979,6 +999,14 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
> > return EXIT_FASTPATH_NONE;
> > }
> > + /*
> > + * Wait until retry of SEPT-zap-related SEAMCALL completes before
> > + * allowing vCPU entry to avoid contention with tdh_vp_enter() and
> > + * TDCALLs.
> > + */
> > + if (unlikely(READ_ONCE(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap)))
> > + return EXIT_FASTPATH_EXIT_HANDLED;
> > +
> > trace_kvm_entry(vcpu, force_immediate_exit);
> > if (pi_test_on(&tdx->pi_desc)) {
> > @@ -1647,15 +1675,23 @@ static int tdx_sept_drop_private_spte(struct kvm *kvm, gfn_t gfn,
> > if (KVM_BUG_ON(!is_hkid_assigned(kvm_tdx), kvm))
> > return -EINVAL;
> > - do {
> > + /*
> > + * When zapping private page, write lock is held. So no race condition
> > + * with other vcpu sept operation.
> > + * Race with TDH.VP.ENTER due to (0-step mitigation) and Guest TDCALLs.
> > + */
> > + err = tdh_mem_page_remove(kvm_tdx->tdr_pa, gpa, tdx_level, &entry,
> > + &level_state);
> > + if ((err & TDX_OPERAND_BUSY)) {
>
> It is not safe to use "err & TDX_OPERAND_BUSY".
> E.g., if the error is TDX_EPT_WALK_FAILED, "err & TDX_OPERAND_BUSY" will be true.
>
> Maybe you can add a helper to check it.
>
> staticinlinebooltdx_operand_busy(u64err)
> {
> return(err &TDX_SEAMCALL_STATUS_MASK) ==TDX_OPERAND_BUSY;
> }
>
Good catch!
Thanks!

>
> > /*
> > - * When zapping private page, write lock is held. So no race
> > - * condition with other vcpu sept operation. Race only with
> > - * TDH.VP.ENTER.
> > + * The second retry is expected to succeed after kicking off all
> > + * other vCPUs and prevent them from invoking TDH.VP.ENTER.
> > */
> > + tdx_no_vcpus_enter_start(kvm);
> > err = tdh_mem_page_remove(kvm_tdx->tdr_pa, gpa, tdx_level, &entry,
> > &level_state);
> > - } while (unlikely(err == TDX_ERROR_SEPT_BUSY));
> > + tdx_no_vcpus_enter_stop(kvm);
> > + }
> > if (unlikely(kvm_tdx->state != TD_STATE_RUNNABLE &&
> > err == (TDX_EPT_WALK_FAILED | TDX_OPERAND_ID_RCX))) {
> > @@ -1726,8 +1762,12 @@ static int tdx_sept_zap_private_spte(struct kvm *kvm, gfn_t gfn,
> > WARN_ON_ONCE(level != PG_LEVEL_4K);
> > err = tdh_mem_range_block(kvm_tdx->tdr_pa, gpa, tdx_level, &entry, &level_state);
> > - if (unlikely(err == TDX_ERROR_SEPT_BUSY))
> > - return -EAGAIN;
> > + if (unlikely(err & TDX_OPERAND_BUSY)) {
> Ditto.
>
> > + /* After no vCPUs enter, the second retry is expected to succeed */
> > + tdx_no_vcpus_enter_start(kvm);
> > + err = tdh_mem_range_block(kvm_tdx->tdr_pa, gpa, tdx_level, &entry, &level_state);
> > + tdx_no_vcpus_enter_stop(kvm);
> > + }
> > if (KVM_BUG_ON(err, kvm)) {
> > pr_tdx_error_2(TDH_MEM_RANGE_BLOCK, err, entry, level_state);
> > return -EIO;
> > @@ -1770,9 +1810,13 @@ static void tdx_track(struct kvm *kvm)
> > lockdep_assert_held_write(&kvm->mmu_lock);
> > - do {
> > + err = tdh_mem_track(kvm_tdx->tdr_pa);
> > + if ((err & TDX_SEAMCALL_STATUS_MASK) == TDX_OPERAND_BUSY) {
> > + /* After no vCPUs enter, the second retry is expected to succeed */
> > + tdx_no_vcpus_enter_start(kvm);
> > err = tdh_mem_track(kvm_tdx->tdr_pa);
> > - } while (unlikely((err & TDX_SEAMCALL_STATUS_MASK) == TDX_OPERAND_BUSY));
> > + tdx_no_vcpus_enter_stop(kvm);
> > + }
> >
> …