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

Binbin Wu
2025-01-16T06:28:10Z

On 1/16/2025 2:23 PM, 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;
> }
>
Don't know why some spaces were dropped by thunderbird. :-(

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