[PATCH RFC 24/39] KVM: x86/xen: backend hypercall support

[Joao Martins]

From: Ankur Arora <ankur.a.arora@xxxxxxxxxx>

Ordinarily a Xen backend domain would do hypercalls via int 0x81 (or
vmcall) to enter a lower ring of execution. This is done via a
hypercall_page which contains call stubs corresponding to each
hypercall.

For Xen backend driver support, however, we would like to do Xen
hypercalls in the same ring. To that end we point the hypercall_page to
a kvm owned text page which just does a local call (to
kvm_xen_host_hcall().)

Note, that this is different from hypercalls handled in
kvm_xen_hypercall(), because the latter refers to domU hypercalls (so
there is an actual drop in execution ring) while there isn't in
kvm_xen_host_hcall().

Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |  3 ++
 arch/x86/kvm/Makefile           |  2 +-
 arch/x86/kvm/xen-asm.S          | 66 +++++++++++++++++++++++++++++++++++++++
 arch/x86/kvm/xen.c              | 68 +++++++++++++++++++++++++++++++++++++++++
 arch/x86/kvm/xen.h              |  4 +++
 5 files changed, 142 insertions(+), 1 deletion(-)
 create mode 100644 arch/x86/kvm/xen-asm.S

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 70bb7339ddd4..55609e919e14 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1669,4 +1669,7 @@ static inline int kvm_cpu_get_apicid(int mps_cpu)
 #define put_smstate(type, buf, offset, val)                      \
 	*(type *)((buf) + (offset) - 0x7e00) = val
 
+void kvm_xen_register_lcall(struct kvm_xen *shim);
+void kvm_xen_unregister_lcall(void);
+
 #endif /* _ASM_X86_KVM_HOST_H */
diff --git a/arch/x86/kvm/Makefile b/arch/x86/kvm/Makefile
index 2b46c93c9380..c1eaabbd0a54 100644
--- a/arch/x86/kvm/Makefile
+++ b/arch/x86/kvm/Makefile
@@ -10,7 +10,7 @@ kvm-$(CONFIG_KVM_ASYNC_PF)	+= $(KVM)/async_pf.o
 
 kvm-y			+= x86.o mmu.o emulate.o i8259.o irq.o lapic.o \
 			   i8254.o ioapic.o irq_comm.o cpuid.o pmu.o mtrr.o \
-			   hyperv.o xen.o page_track.o debugfs.o
+			   hyperv.o xen-asm.o xen.o page_track.o debugfs.o
 
 kvm-intel-y		+= vmx/vmx.o vmx/vmenter.o vmx/pmu_intel.o vmx/vmcs12.o vmx/evmcs.o vmx/nested.o
 kvm-amd-y		+= svm.o pmu_amd.o
diff --git a/arch/x86/kvm/xen-asm.S b/arch/x86/kvm/xen-asm.S
new file mode 100644
index 000000000000..10559fcfbe38
--- /dev/null
+++ b/arch/x86/kvm/xen-asm.S
@@ -0,0 +1,66 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/* Copyright (c) 2019 Oracle and/or its affiliates. All rights reserved. */
+#include <linux/linkage.h>
+#include <asm/asm.h>
+#include <asm/frame.h>
+#include <asm/page_types.h>
+#include <asm/unwind_hints.h>
+#include <xen/interface/xen.h>
+#include <xen/interface/xen-mca.h>
+#include <asm/xen/interface.h>
+
+	.balign PAGE_SIZE
+ENTRY(kvm_xen_hypercall_page)
+	hcall=0
+	.rept (PAGE_SIZE / 32)
+		FRAME_BEGIN
+		push %rcx	/* Push call clobbered registers */
+		push %r9
+		push %r11
+		mov $hcall, %rax
+
+		call kvm_xen_host_hcall
+		pop %r11
+		pop %r9
+		pop %rcx
+
+		FRAME_END
+		ret
+		.balign 32
+		hcall = hcall + 1
+	.endr
+/*
+ * Hypercall symbols are used for unwinding the stack, so we give them names
+ * prefixed with kvm_xen_ (Xen hypercalls have symbols prefixed with xen_.)
+ */
+#define HYPERCALL(n) \
+  .equ kvm_xen_hypercall_##n, kvm_xen_hypercall_page + __HYPERVISOR_##n * 32; \
+  .type kvm_xen_hypercall_##n, @function;				      \
+  .size kvm_xen_hypercall_##n, 32
+#include <asm/xen-hypercalls.h>
+#undef HYPERCALL
+END(kvm_xen_hypercall_page)
+
+/*
+ * Some call stubs generated above do not have associated symbols. Generate
+ * bogus symbols for those hypercall blocks to stop objtool from complaining
+ * about unreachable code.
+ */
+.altmacro
+.macro hypercall_missing N
+       .equ kvm_xen_hypercall_missing_\N, kvm_xen_hypercall_page + \N * 32;
+       .type kvm_xen_hypercall_missing_\N, @function;
+       .size kvm_xen_hypercall_missing_\N, 32;
+.endm
+
+.macro hypercalls_missing N count=1
+       .set n,\N
+       .rept \count
+               hypercall_missing %n
+               .set n,n+1
+       .endr
+.endm
+
+hypercalls_missing 11 1
+hypercalls_missing 42 6
+hypercalls_missing 56 72
diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 7266d27db210..645cd22ab4e7 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -12,6 +12,7 @@
 #include <linux/kvm_host.h>
 #include <linux/eventfd.h>
 #include <linux/sched/stat.h>
+#include <linux/linkage.h>
 
 #include <trace/events/kvm.h>
 #include <xen/interface/xen.h>
@@ -19,6 +20,10 @@
 #include <xen/interface/event_channel.h>
 #include <xen/interface/grant_table.h>
 #include <xen/interface/sched.h>
+#include <xen/interface/version.h>
+#include <xen/xen.h>
+#include <xen/features.h>
+#include <asm/xen/hypercall.h>
 
 #include "trace.h"
 
@@ -43,6 +48,7 @@ struct evtchnfd {
 static int kvm_xen_evtchn_send(struct kvm_vcpu *vcpu, int port);
 static void *xen_vcpu_info(struct kvm_vcpu *v);
 static void kvm_xen_gnttab_free(struct kvm_xen *xen);
+static int shim_hypercall(u64 code, u64 a0, u64 a1, u64 a2, u64 a3, u64 a4);
 
 #define XEN_DOMID_MIN	1
 #define XEN_DOMID_MAX	(DOMID_FIRST_RESERVED - 1)
@@ -50,6 +56,9 @@ static void kvm_xen_gnttab_free(struct kvm_xen *xen);
 static rwlock_t domid_lock;
 static struct idr domid_to_kvm;
 
+static struct hypercall_entry *hypercall_page_save;
+static struct kvm_xen *xen_shim __read_mostly;
+
 static int kvm_xen_domid_init(struct kvm *kvm, bool any, domid_t domid)
 {
 	u16 min = XEN_DOMID_MIN, max = XEN_DOMID_MAX;
@@ -1271,3 +1280,62 @@ int kvm_vm_ioctl_xen_gnttab(struct kvm *kvm, struct kvm_xen_gnttab *op)
 
 	return r;
 }
+
+asmlinkage int kvm_xen_host_hcall(void)
+{
+	register unsigned long a0 asm(__HYPERCALL_RETREG);
+	register unsigned long a1 asm(__HYPERCALL_ARG1REG);
+	register unsigned long a2 asm(__HYPERCALL_ARG2REG);
+	register unsigned long a3 asm(__HYPERCALL_ARG3REG);
+	register unsigned long a4 asm(__HYPERCALL_ARG4REG);
+	register unsigned long a5 asm(__HYPERCALL_ARG5REG);
+	int ret;
+
+	preempt_disable();
+	ret = shim_hypercall(a0, a1, a2, a3, a4, a5);
+	preempt_enable();
+
+	return ret;
+}
+
+void kvm_xen_register_lcall(struct kvm_xen *shim)
+{
+	hypercall_page_save = hypercall_page;
+	hypercall_page = kvm_xen_hypercall_page;
+	xen_shim = shim;
+}
+EXPORT_SYMBOL_GPL(kvm_xen_register_lcall);
+
+void kvm_xen_unregister_lcall(void)
+{
+	hypercall_page = hypercall_page_save;
+	hypercall_page_save = NULL;
+}
+EXPORT_SYMBOL_GPL(kvm_xen_unregister_lcall);
+
+static int shim_hcall_version(int op, struct xen_feature_info *fi)
+{
+	if (op != XENVER_get_features || !fi || fi->submap_idx != 0)
+		return -EINVAL;
+
+	/*
+	 * We need a limited set of features for a pseudo dom0.
+	 */
+	fi->submap = (1U << XENFEAT_auto_translated_physmap);
+	return 0;
+}
+
+static int shim_hypercall(u64 code, u64 a0, u64 a1, u64 a2, u64 a3, u64 a4)
+{
+	int ret = -ENOSYS;
+
+	switch (code) {
+	case __HYPERVISOR_xen_version:
+		ret = shim_hcall_version((int)a0, (void *)a1);
+		break;
+	default:
+		break;
+	}
+
+	return ret;
+}
diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
index 08ad4e1259df..9fa7c3dd111a 100644
--- a/arch/x86/kvm/xen.h
+++ b/arch/x86/kvm/xen.h
@@ -3,6 +3,8 @@
 #ifndef __ARCH_X86_KVM_XEN_H__
 #define __ARCH_X86_KVM_XEN_H__
 
+#include <asm/xen/hypercall.h>
+
 static inline struct kvm_vcpu_xen *vcpu_to_xen_vcpu(struct kvm_vcpu *vcpu)
 {
 	return &vcpu->arch.xen;
@@ -48,4 +50,6 @@ int kvm_xen_has_pending_timer(struct kvm_vcpu *vcpu);
 void kvm_xen_inject_timer_irqs(struct kvm_vcpu *vcpu);
 bool kvm_xen_timer_enabled(struct kvm_vcpu *vcpu);
 
+extern struct hypercall_entry kvm_xen_hypercall_page[128];
+
 #endif
-- 
2.11.0