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authorOliver Upton2022-03-16 00:55:37 +0000
committerPaolo Bonzini2022-04-02 05:41:10 -0400
commitf1a9761fbb00639c5e73835f7f373ef834bb867f (patch)
tree423a1f817617a7cc72d059ab2a83639469a7034f /Documentation/virt
parentc15e0ae42c8e5a61e9aca8aac920517cf7b3e94e (diff)
KVM: x86: Allow userspace to opt out of hypercall patching
KVM handles the VMCALL/VMMCALL instructions very strangely. Even though both of these instructions really should #UD when executed on the wrong vendor's hardware (i.e. VMCALL on SVM, VMMCALL on VMX), KVM replaces the guest's instruction with the appropriate instruction for the vendor. Nonetheless, older guest kernels without commit c1118b3602c2 ("x86: kvm: use alternatives for VMCALL vs. VMMCALL if kernel text is read-only") do not patch in the appropriate instruction using alternatives, likely motivating KVM's intervention. Add a quirk allowing userspace to opt out of hypercall patching. If the quirk is disabled, KVM synthesizes a #UD in the guest. Signed-off-by: Oliver Upton <oupton@google.com> Message-Id: <20220316005538.2282772-2-oupton@google.com> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Diffstat (limited to 'Documentation/virt')
-rw-r--r--Documentation/virt/kvm/api.rst9
1 files changed, 9 insertions, 0 deletions
diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index d13fa6600467..700df350332b 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7134,6 +7134,15 @@ The valid bits in cap.args[0] are:
Additionally, when this quirk is disabled,
KVM clears CPUID.01H:ECX[bit 3] if
IA32_MISC_ENABLE[bit 18] is cleared.
+
+ KVM_X86_QUIRK_FIX_HYPERCALL_INSN By default, KVM rewrites guest
+ VMMCALL/VMCALL instructions to match the
+ vendor's hypercall instruction for the
+ system. When this quirk is disabled, KVM
+ will no longer rewrite invalid guest
+ hypercall instructions. Executing the
+ incorrect hypercall instruction will
+ generate a #UD within the guest.
=================================== ============================================
8. Other capabilities.