[PATCH 03/11] KVM: x86/xen: Don't truncate RAX when handling hypercall from protected guest

Sean Christopherson
2026-04-09T23:56:34Z

Don't truncate RAX when handling a Xen hypercall for a guest with protected
state, as KVM's ABI is to assume the guest is in 64-bit for such cases
(the guest leaving garbage in 63:32 after a transition to 32-bit mode is
far less likely than 63:32 being necessary to complete the hypercall).

Fixes: b5aead0064f3 ("KVM: x86: Assume a 64-bit hypercall for guests with protected state")
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/xen.c | 6 +++---
1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 6d9be74bb673..895095dc684e 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1678,15 +1678,14 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
bool handled = false;
u8 cpl;

- input = (u64)kvm_register_read(vcpu, VCPU_REGS_RAX);
-
/* Hyper-V hypercalls get bit 31 set in EAX */
- if ((input & 0x80000000) &&
+ if ((kvm_rax_read(vcpu) & 0x80000000) &&
kvm_hv_hypercall_enabled(vcpu))
return kvm_hv_hypercall(vcpu);

longmode = is_64_bit_hypercall(vcpu);
if (!longmode) {
+ input = (u32)kvm_rax_read(vcpu);
params[0] = (u32)kvm_rbx_read(vcpu);
params[1] = (u32)kvm_rcx_read(vcpu);
params[2] = (u32)kvm_rdx_read(vcpu);
@@ -1696,6 +1695,7 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
}
else {
#ifdef CONFIG_X86_64
+ input = (u64)kvm_rax_read(vcpu);
params[0] = (u64)kvm_rdi_read(vcpu);
params[1] = (u64)kvm_rsi_read(vcpu);
params[2] = (u64)kvm_rdx_read(vcpu);
--
2.53.0.1213.gd9a14994de-goog