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

Binbin Wu
2025-09-22T07:45:52Z

On 9/20/2025 6:32 AM, Sean Christopherson wrote:
> 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

Writable=0,Dirty=0 -> Writable=0,Dirty=1

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

> 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),