[PATCH 03/15] KVM: x86/mmu: Introduce exception flag for unmapped GPAs

Nikolas Wipper
2024-09-10T15:23:08Z

Introduce a flag in x86_exception which signals that a page walk failed
because a page table GPA wasn't backed by a memslot. This only applies to
page tables; the final physical address is not checked.

This extra flag is needed, because the normal page fault error code does
not contain a bit to signal this kind of fault.

Used in subsequent patches to give userspace information about translation
failure.

Signed-off-by: Nikolas Wipper <nikwip@amazon.de>
---
arch/x86/kvm/kvm_emulate.h | 2 ++
arch/x86/kvm/mmu/paging_tmpl.h | 6 +++++-
2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/kvm_emulate.h b/arch/x86/kvm/kvm_emulate.h
index 55a18e2f2dcd..afd8e86bc6af 100644
--- a/arch/x86/kvm/kvm_emulate.h
+++ b/arch/x86/kvm/kvm_emulate.h
@@ -27,6 +27,8 @@ struct x86_exception {
u64 address; /* cr2 or nested page fault gpa */
u8 async_page_fault;
unsigned long exit_qualification;
+#define KVM_X86_UNMAPPED_PTE_GPA BIT(0)
+ u16 flags;
};

/*
diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
index d9c3c78b3c14..f6a78b7cfca1 100644
--- a/arch/x86/kvm/mmu/paging_tmpl.h
+++ b/arch/x86/kvm/mmu/paging_tmpl.h
@@ -339,6 +339,8 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
#endif
walker->max_level = walker->level;

+ walker->fault.flags = 0;
+
/*
* FIXME: on Intel processors, loads of the PDPTE registers for PAE paging
* by the MOV to CR instruction are treated as reads and do not cause the
@@ -393,8 +395,10 @@ static int FNAME(walk_addr_generic)(struct guest_walker *walker,
return 0;

slot = kvm_vcpu_gfn_to_memslot(vcpu, gpa_to_gfn(real_gpa));
- if (!kvm_is_visible_memslot(slot))
+ if (!kvm_is_visible_memslot(slot)) {
+ walker->fault.flags = KVM_X86_UNMAPPED_PTE_GPA;
goto error;
+ }

host_addr = gfn_to_hva_memslot_prot(slot, gpa_to_gfn(real_gpa),
&walker->pte_writable[walker->level - 1]);