KVM: x86: Redefine 'longmode' as a flag for KVM_EXIT_HYPERCALL

The 'longmode' field is a bit annoying as it blows an entire __u32 to
represent a boolean value. Since other architectures are looking to add
support for KVM_EXIT_HYPERCALL, now is probably a good time to clean it
up.

Redefine the field (and the remaining padding) as a set of flags.
Preserve the existing ABI by using bit 0 to indicate if the guest was in
long mode and requiring that the remaining 31 bits must be zero.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20230404154050.2270077-2-oliver.upton@linux.dev

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 62de076..9b01e3d 100644 (file)
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -6218,8 +6218,7 @@ to the byte array.
                        __u64 nr;
                        __u64 args[6];
                        __u64 ret;
-                       __u32 longmode;
-                       __u32 pad;
+                       __u64 flags;
                } hypercall;
 
 Unused.  This was once used for 'hypercall to userspace'.  To implement

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 808c292..15bda40 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -2204,4 +2204,11 @@ int memslot_rmap_alloc(struct kvm_memory_slot *slot, unsigned long npages);
         KVM_X86_QUIRK_FIX_HYPERCALL_INSN |     \
         KVM_X86_QUIRK_MWAIT_NEVER_UD_FAULTS)
 
+/*
+ * KVM previously used a u32 field in kvm_run to indicate the hypercall was
+ * initiated from long mode. KVM now sets bit 0 to indicate long mode, but the
+ * remaining 31 lower bits must be 0 to preserve ABI.
+ */
+#define KVM_EXIT_HYPERCALL_MBZ         GENMASK_ULL(31, 1)
+
 #endif /* _ASM_X86_KVM_HOST_H */

diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h
index 7f467fe..1a6a1f9 100644 (file)
--- a/arch/x86/include/uapi/asm/kvm.h
+++ b/arch/x86/include/uapi/asm/kvm.h
@@ -559,4 +559,7 @@ struct kvm_pmu_event_filter {
 #define KVM_VCPU_TSC_CTRL 0 /* control group for the timestamp counter (TSC) */
 #define   KVM_VCPU_TSC_OFFSET 0 /* attribute for the TSC offset */
 
+/* x86-specific KVM_EXIT_HYPERCALL flags. */
+#define KVM_EXIT_HYPERCALL_LONG_MODE   BIT(0)
+
 #endif /* _ASM_X86_KVM_H */

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7713420..27a1d5c 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -9803,7 +9803,11 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
                vcpu->run->hypercall.args[0]  = gpa;
                vcpu->run->hypercall.args[1]  = npages;
                vcpu->run->hypercall.args[2]  = attrs;
-               vcpu->run->hypercall.longmode = op_64_bit;
+               vcpu->run->hypercall.flags    = 0;
+               if (op_64_bit)
+                       vcpu->run->hypercall.flags |= KVM_EXIT_HYPERCALL_LONG_MODE;
+
+               WARN_ON_ONCE(vcpu->run->hypercall.flags & KVM_EXIT_HYPERCALL_MBZ);
                vcpu->arch.complete_userspace_io = complete_hypercall_exit;
                return 0;
        }

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index d77aef8..dd42d7d 100644 (file)
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -341,8 +341,13 @@ struct kvm_run {
                        __u64 nr;
                        __u64 args[6];
                        __u64 ret;
-                       __u32 longmode;
-                       __u32 pad;
+
+                       union {
+#ifndef __KERNEL__
+                               __u32 longmode;
+#endif
+                               __u64 flags;
+                       };
                } hypercall;
                /* KVM_EXIT_TPR_ACCESS */
                struct {