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Thu, 16 Oct 2025 17:33:09 -0700 (PDT) Reply-To: Sean Christopherson Date: Thu, 16 Oct 2025 17:32:30 -0700 In-Reply-To: <20251017003244.186495-1-seanjc@google.com> X-Mailing-List: linuxppc-dev@lists.ozlabs.org List-Id: List-Help: List-Owner: List-Post: List-Archive: , List-Subscribe: , , List-Unsubscribe: Precedence: list Mime-Version: 1.0 References: <20251017003244.186495-1-seanjc@google.com> X-Mailer: git-send-email 2.51.0.858.gf9c4a03a3a-goog Message-ID: <20251017003244.186495-13-seanjc@google.com> Subject: [PATCH v3 12/25] KVM: TDX: Use atomic64_dec_return() instead of a poor equivalent From: Sean Christopherson To: Marc Zyngier , Oliver Upton , Tianrui Zhao , Bibo Mao , Huacai Chen , Madhavan Srinivasan , Anup Patel , Paul Walmsley , Palmer Dabbelt , Albert Ou , Christian Borntraeger , Janosch Frank , Claudio Imbrenda , Sean Christopherson , Paolo Bonzini , "Kirill A. Shutemov" Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, loongarch@lists.linux.dev, linux-mips@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, kvm-riscv@lists.infradead.org, linux-riscv@lists.infradead.org, x86@kernel.org, linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org, Ira Weiny , Kai Huang , Michael Roth , Yan Zhao , Vishal Annapurve , Rick Edgecombe , Ackerley Tng , Binbin Wu Content-Type: text/plain; charset="UTF-8" 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 Reviewed-by: Ira Weiny Reviewed-by: Binbin Wu Signed-off-by: Sean Christopherson --- 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 220989a1e085..6c0adc1b3bd5 100644 --- a/arch/x86/kvm/vmx/tdx.c +++ b/arch/x86/kvm/vmx/tdx.c @@ -1722,10 +1722,9 @@ 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; } @@ -3157,8 +3156,7 @@ 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) { -- 2.51.0.858.gf9c4a03a3a-goog