Re: [RFC PATCH v2 10/18] KVM: TDX: Use atomic64_dec_return() instead of a poor equivalent

[Binbin Wu <binbin.wu@linux.intel.com>]

On 8/29/2025 8:06 AM, Sean Christopherson wrote:
> Use atomic64_dec_return() when decrementing the number of "pre-mapped"
> S-EPT pages to ensure that the count can't go negative without KVM
> noticing.  In theory, checking for '0' and then decrementing in a separate
> operation could miss a 0=>-1 transition.  In practice, such a condition is
> impossible because nr_premapped is protected by slots_lock, i.e. doesn't
> actually need to be an atomic (that wart will be addressed shortly).
>
> Don't bother trying to keep the count non-negative, as the KVM_BUG_ON()
> ensures the VM is dead, i.e. there's no point in trying to limp along.
>
> Reviewed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
> Reviewed-by: Ira Weiny <ira.weiny@intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>

> ---
>   arch/x86/kvm/vmx/tdx.c | 6 ++----
>   1 file changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index cafd618ca43c..fe0815d542e3 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1725,10 +1725,9 @@ static int tdx_sept_zap_private_spte(struct kvm *kvm, gfn_t gfn,
>   		tdx_no_vcpus_enter_stop(kvm);
>   	}
>   	if (tdx_is_sept_zap_err_due_to_premap(kvm_tdx, err, entry, level)) {
> -		if (KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm))
> +		if (KVM_BUG_ON(atomic64_dec_return(&kvm_tdx->nr_premapped) < 0, kvm))
>   			return -EIO;
>   
> -		atomic64_dec(&kvm_tdx->nr_premapped);
>   		return 0;
>   	}
>   
> @@ -3151,8 +3150,7 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
>   		goto out;
>   	}
>   
> -	if (!KVM_BUG_ON(!atomic64_read(&kvm_tdx->nr_premapped), kvm))
> -		atomic64_dec(&kvm_tdx->nr_premapped);
> +	KVM_BUG_ON(atomic64_dec_return(&kvm_tdx->nr_premapped) < 0, kvm);
>   
>   	if (arg->flags & KVM_TDX_MEASURE_MEMORY_REGION) {
>   		for (i = 0; i < PAGE_SIZE; i += TDX_EXTENDMR_CHUNKSIZE) {