Re: [PATCH 05/21] KVM: VMX: Teach EPT violation helper about private mem

[Chao Gao]

On Tue, Sep 03, 2024 at 08:07:35PM -0700, Rick Edgecombe wrote:
>Teach EPT violation helper to check shared mask of a GPA to find out
>whether the GPA is for private memory.
>
>When EPT violation is triggered after TD accessing a private GPA, KVM will
>exit to user space if the corresponding GFN's attribute is not private.
>User space will then update GFN's attribute during its memory conversion
>process. After that, TD will re-access the private GPA and trigger EPT
>violation again. Only with GFN's attribute matches to private, KVM will
>fault in private page, map it in mirrored TDP root, and propagate changes
>to private EPT to resolve the EPT violation.
>
>Relying on GFN's attribute tracking xarray to determine if a GFN is
>private, as for KVM_X86_SW_PROTECTED_VM, may lead to endless EPT
>violations.
>
>Co-developed-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
>Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
>Signed-off-by: Rick Edgecombe <rick.p.edgecombe@xxxxxxxxx>
>---
>TDX MMU part 2 v1:
> - Split from "KVM: TDX: handle ept violation/misconfig exit"
>---
> arch/x86/kvm/vmx/common.h | 13 +++++++++++++
> 1 file changed, 13 insertions(+)
>
>diff --git a/arch/x86/kvm/vmx/common.h b/arch/x86/kvm/vmx/common.h
>index 78ae39b6cdcd..10aa12d45097 100644
>--- a/arch/x86/kvm/vmx/common.h
>+++ b/arch/x86/kvm/vmx/common.h
>@@ -6,6 +6,12 @@
> 
> #include "mmu.h"
> 
>+static inline bool kvm_is_private_gpa(struct kvm *kvm, gpa_t gpa)
>+{
>+	/* For TDX the direct mask is the shared mask. */
>+	return !kvm_is_addr_direct(kvm, gpa);
>+}
>+
> static inline int __vmx_handle_ept_violation(struct kvm_vcpu *vcpu, gpa_t gpa,
> 					     unsigned long exit_qualification)
> {
>@@ -28,6 +34,13 @@ static inline int __vmx_handle_ept_violation(struct kvm_vcpu *vcpu, gpa_t gpa,
> 		error_code |= (exit_qualification & EPT_VIOLATION_GVA_TRANSLATED) ?
> 			      PFERR_GUEST_FINAL_MASK : PFERR_GUEST_PAGE_MASK;
> 
>+	/*
>+	 * Don't rely on GFN's attribute tracking xarray to prevent EPT violation
>+	 * loops.
>+	 */

The comment seems a bit odd to me. We cannot use the gfn attribute from the
attribute xarray simply because here we need to determine if *this access* is
to private memory, which may not match the gfn attribute. Even if there are
other ways to prevent an infinite EPT violation loop, we still need to check
the shared bit in the faulting GPA.

>+	if (kvm_is_private_gpa(vcpu->kvm, gpa))
>+		error_code |= PFERR_PRIVATE_ACCESS;
>+
> 	return kvm_mmu_page_fault(vcpu, gpa, error_code, NULL, 0);
> }
> 
>-- 
>2.34.1
>
>