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KVM: TDX: Use atomic64_dec_return() instead of a poor equivalent

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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>
Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Yan Zhao <yan.y.zhao@intel.com>
Tested-by: Kai Huang <kai.huang@intel.com>
Link: https://patch.msgid.link/20251030200951.3402865-15-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>
This commit is contained in:
Sean Christopherson
2025-10-30 13:09:37 -07:00
parent 24adff3970
commit af96d5452e
1 changed files with 2 additions and 4 deletions
Download Patch File Download Diff File Expand all files Collapse all files

6
arch/x86/kvm/vmx/tdx.c
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@@ -1726,10 +1726,9 @@ 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;
}
@@ -3171,8 +3170,7 @@ 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) {