[PATCH 02/11] KVM: x86/xen: Bug the VM if 32-bit KVM observes a 64-bit mode hypercall

Sean Christopherson
2026-04-09T23:56:32Z

Bug the VM if 32-bit KVM attempts to handle a 64-bit hypercall, primarily
so that a future change to set "input" in mode-specific code doesn't
trigger a false positive warn=>error:

arch/x86/kvm/xen.c:1687:6: error: variable 'input' is used uninitialized
whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
1687 | if (!longmode) {
| ^~~~~~~~~
arch/x86/kvm/xen.c:1708:31: note: uninitialized use occurs here
1708 | trace_kvm_xen_hypercall(cpl, input, params[0], params[1], params[2],
| ^~~~~
x86/kvm/xen.c:1687:2: note: remove the 'if' if its condition is always true
1687 | if (!longmode) {
| ^~~~~~~~~~~~~~
arch/x86/kvm/xen.c:1677:11: note: initialize the variable 'input' to silence this warning
1677 | u64 input, params[6], r = -ENOSYS;
| ^
1 error generated.

Note, params[] also has the same flaw, but -Wsometimes-uninitialized
doesn't seem to be enforced for arrays, presumably because it's difficult
to avoid false positives on specific entries.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/xen.c | 7 +++++--
1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 91fd3673c09a..6d9be74bb673 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1694,16 +1694,19 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
params[4] = (u32)kvm_rdi_read(vcpu);
params[5] = (u32)kvm_rbp_read(vcpu);
}
-#ifdef CONFIG_X86_64
else {
+#ifdef CONFIG_X86_64
params[0] = (u64)kvm_rdi_read(vcpu);
params[1] = (u64)kvm_rsi_read(vcpu);
params[2] = (u64)kvm_rdx_read(vcpu);
params[3] = (u64)kvm_r10_read(vcpu);
params[4] = (u64)kvm_r8_read(vcpu);
params[5] = (u64)kvm_r9_read(vcpu);
- }
+#else
+ KVM_BUG_ON(1, vcpu->kvm);
+ return -EIO;
#endif
+ }
cpl = kvm_x86_call(get_cpl)(vcpu);
trace_kvm_xen_hypercall(cpl, input, params[0], params[1], params[2],
params[3], params[4], params[5]);
--
2.53.0.1213.gd9a14994de-goog