Re: [PATCH v3 01/13] KVM: x86/xen: Rename 'longmode' to 'is_64bit' in hypercall handling

[Sean Christopherson <seanjc@google.com>]

On Wed, Sep 02, 2026, David Woodhouse wrote:
> On Wed, 2026-09-02 at 13:18 +0100, Paul Durrant wrote:
> > On 31/08/2026 22:26, David Woodhouse wrote:
> > > From: David Woodhouse <dwmw@amazon.co.uk>
> > > 
> > > Rename the local 'longmode' variable and function parameter to
> > > 'is_64bit' throughout the Xen hypercall handling code. This
> > > distinguishes it from the VM-wide kvm->arch.xen.long_mode which
> > > represents the Xen shared_info layout mode.
> > > 
> > > The 'is_64bit' parameter indicates whether the vCPU was in 64-bit
> > > mode when it made the hypercall, which determines how to parse the
> > > hypercall arguments. The UAPI field name (vcpu->run->xen.u.hcall.longmode)
> > > is unchanged.
> > > 
> > 
> > Given that 'longmode' is the term used in the UAPI I'm not sure I really 
> > see the point in this change (particularly since there is not even a 
> > name clash with 'long_mode').
> 
> The difference between 'longmode' and 'long_mode' is subtle, and *has*
> caused confusion which IIRC is what led to part of this series.
> 
> Having to keep 'longmode' in the UAPI for KVM_EXIT_XEN_HCALL is sad,
> but at least the context is very clear there (xen.u.hcall.longmode).

We can actually "fix" that, if we want.  And given that the only "longmode"
reference left in KVM is one in kvm_hv_hypercall_set_result() that can and should
be nuked, I think it make sense to purge longmode from KVM's source.

We already did something very similar in e65733b5c59a ("KVM: x86: Redefine 'longmode'
as a flag for KVM_EXIT_HYPERCALL").  And if we expose both names to userspace, we can
even purge the misleading name from selftests without forcing existing VMMs to
rebuild.

E.g. (completely untested)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 718396340d3c..043a61e2409d 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1804,7 +1804,7 @@ int kvm_xen_hypercall(struct kvm_vcpu *vcpu)
 handle_in_userspace:
 	vcpu->run->exit_reason = KVM_EXIT_XEN;
 	vcpu->run->xen.type = KVM_EXIT_XEN_HCALL;
-	vcpu->run->xen.u.hcall.longmode = is_64bit;
+	vcpu->run->xen.u.hcall.is_64bit = is_64bit;
 	vcpu->run->xen.u.hcall.cpl = cpl;
 	vcpu->run->xen.u.hcall.input = input;
 	vcpu->run->xen.u.hcall.params[0] = params[0];
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 9fc8dfdfd65f..5a74765732e3 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -131,7 +131,12 @@ struct kvm_xen_exit {
 	__u32 type;
 	union {
 		struct {
-			__u32 longmode;
+			union {
+#ifndef __KERNEL__
+				__u32 longmode;
+#endif
+				__u32 is_64bit;
+			};
 			__u32 cpl;
 			__u64 input;
 			__u64 result;
diff --git a/tools/testing/selftests/kvm/x86/xen_vmcall_test.c b/tools/testing/selftests/kvm/x86/xen_vmcall_test.c
index 2585087cdf5c..702920674b57 100644
--- a/tools/testing/selftests/kvm/x86/xen_vmcall_test.c
+++ b/tools/testing/selftests/kvm/x86/xen_vmcall_test.c
@@ -111,7 +111,7 @@ int main(int argc, char *argv[])
 		if (run->exit_reason == KVM_EXIT_XEN) {
 			TEST_ASSERT_EQ(run->xen.type, KVM_EXIT_XEN_HCALL);
 			TEST_ASSERT_EQ(run->xen.u.hcall.cpl, 0);
-			TEST_ASSERT_EQ(run->xen.u.hcall.longmode, 1);
+			TEST_ASSERT_EQ(run->xen.u.hcall.is_64bit, 1);
 			TEST_ASSERT_EQ(run->xen.u.hcall.input, INPUTVALUE);
 			TEST_ASSERT_EQ(run->xen.u.hcall.params[0], ARGVALUE(1));
 			TEST_ASSERT_EQ(run->xen.u.hcall.params[1], ARGVALUE(2));