[PATCH v16 26/51] KVM: x86: Disable support for Shadow Stacks if TDP is disabled

Sean Christopherson
2025-09-19T22:33:51Z

Make TDP a hard requirement for Shadow Stacks, as there are no plans to
add Shadow Stack support to the Shadow MMU. E.g. KVM hasn't been taught
to understand the magic Writable=0,Dirty=0 combination that is required
for Shadow Stack accesses, and so enabling Shadow Stacks when using
shadow paging will put the guest into an infinite #PF loop (KVM thinks the
shadow page tables have a valid mapping, hardware says otherwise).

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/cpuid.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 32fde9e80c28..499c86bd457e 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -955,6 +955,14 @@ void kvm_set_cpu_caps(void)
if (!tdp_enabled || !boot_cpu_has(X86_FEATURE_OSPKE))
kvm_cpu_cap_clear(X86_FEATURE_PKU);

+ /*
+ * Shadow Stacks aren't implemented in the Shadow MMU. Shadow Stack
+ * accesses require "magic" Writable=0,Dirty=1 protection, which KVM
+ * doesn't know how to emulate or map.
+ */
+ if (!tdp_enabled)
+ kvm_cpu_cap_clear(X86_FEATURE_SHSTK);
+
kvm_cpu_cap_init(CPUID_7_EDX,
F(AVX512_4VNNIW),
F(AVX512_4FMAPS),
--
2.51.0.470.ga7dc726c21-goog