[RFC 05/33] KVM: x86: hyper-v: Introduce VTL call/return prologues in hypercall page

[Nicolas Saenz Julienne <nsaenz@amazon.com>]

VTL call/return hypercalls have their own entry points in the hypercall
page because they don't follow normal hyper-v hypercall conventions.
Move the VTL call/return control input into ECX/RAX and set the
hypercall code into EAX/RCX before calling the hypercall instruction in
order to be able to use the Hyper-V hypercall entry function.

Guests can read an emulated code page offsets register to know the
offsets into the hypercall page for the VTL call/return entries.

Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>

---

My tree has the additional patch, we're still trying to understand under
what conditions Windows expects the offset to be fixed.

diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 54f7f36a89bf..9f2ea8c34447 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -294,6 +294,7 @@ static int patch_hypercall_page(struct kvm_vcpu *vcpu, u64 data)

        /* VTL call/return entries */
        if (!kvm_xen_hypercall_enabled(kvm) && kvm_hv_vsm_enabled(kvm)) {
+               i = 22;
 #ifdef CONFIG_X86_64
                if (is_64_bit_mode(vcpu)) {
                        /*
---
 arch/x86/include/asm/kvm_host.h   |  2 +
 arch/x86/kvm/hyperv.c             | 78 ++++++++++++++++++++++++++++++-
 include/asm-generic/hyperv-tlfs.h | 11 +++++
 3 files changed, 90 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index a2f224f95404..00cd21b09f8c 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1105,6 +1105,8 @@ struct kvm_hv {
 	u64 hv_tsc_emulation_status;
 	u64 hv_invtsc_control;
 
+	union hv_register_vsm_code_page_offsets vsm_code_page_offsets;
+
 	/* How many vCPUs have VP index != vCPU index */
 	atomic_t num_mismatched_vp_indexes;
 
diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
index 78d053042667..d4b1b53ea63d 100644
--- a/arch/x86/kvm/hyperv.c
+++ b/arch/x86/kvm/hyperv.c
@@ -259,7 +259,8 @@ static void synic_exit(struct kvm_vcpu_hv_synic *synic, u32 msr)
 static int patch_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
 {
 	struct kvm *kvm = vcpu->kvm;
-	u8 instructions[9];
+	struct kvm_hv *hv = to_kvm_hv(kvm);
+	u8 instructions[0x30];
 	int i = 0;
 	u64 addr;
 
@@ -285,6 +286,81 @@ static int patch_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
 	/* ret */
 	((unsigned char *)instructions)[i++] = 0xc3;
 
+	/* VTL call/return entries */
+	if (!kvm_xen_hypercall_enabled(kvm) && kvm_hv_vsm_enabled(kvm)) {
+#ifdef CONFIG_X86_64
+		if (is_64_bit_mode(vcpu)) {
+			/*
+			 * VTL call 64-bit entry prologue:
+			 * 	mov %rcx, %rax
+			 * 	mov $0x11, %ecx
+			 * 	jmp 0:
+			 */
+			hv->vsm_code_page_offsets.vtl_call_offset = i;
+			instructions[i++] = 0x48;
+			instructions[i++] = 0x89;
+			instructions[i++] = 0xc8;
+			instructions[i++] = 0xb9;
+			instructions[i++] = 0x11;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0xeb;
+			instructions[i++] = 0xe0;
+			/*
+			 * VTL return 64-bit entry prologue:
+			 * 	mov %rcx, %rax
+			 * 	mov $0x12, %ecx
+			 * 	jmp 0:
+			 */
+			hv->vsm_code_page_offsets.vtl_return_offset = i;
+			instructions[i++] = 0x48;
+			instructions[i++] = 0x89;
+			instructions[i++] = 0xc8;
+			instructions[i++] = 0xb9;
+			instructions[i++] = 0x12;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0xeb;
+			instructions[i++] = 0xd6;
+		} else
+#endif
+		{
+			/*
+			 * VTL call 32-bit entry prologue:
+			 * 	mov %eax, %ecx
+			 * 	mov $0x11, %eax
+			 * 	jmp 0:
+			 */
+			hv->vsm_code_page_offsets.vtl_call_offset = i;
+			instructions[i++] = 0x89;
+			instructions[i++] = 0xc1;
+			instructions[i++] = 0xb8;
+			instructions[i++] = 0x11;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0xeb;
+			instructions[i++] = 0xf3;
+			/*
+			 * VTL return 32-bit entry prologue:
+			 * 	mov %eax, %ecx
+			 * 	mov $0x12, %eax
+			 * 	jmp 0:
+			 */
+			hv->vsm_code_page_offsets.vtl_return_offset = i;
+			instructions[i++] = 0x89;
+			instructions[i++] = 0xc1;
+			instructions[i++] = 0xb8;
+			instructions[i++] = 0x12;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0x00;
+			instructions[i++] = 0xeb;
+			instructions[i++] = 0xea;
+		}
+	}
 	addr = data & HV_X64_MSR_HYPERCALL_PAGE_ADDRESS_MASK;
 	if (kvm_vcpu_write_guest(vcpu, addr, instructions, i))
 		return 1;
diff --git a/include/asm-generic/hyperv-tlfs.h b/include/asm-generic/hyperv-tlfs.h
index fdac4a1714ec..0e7643c1ef01 100644
--- a/include/asm-generic/hyperv-tlfs.h
+++ b/include/asm-generic/hyperv-tlfs.h
@@ -823,4 +823,15 @@ struct hv_mmio_write_input {
 	u8 data[HV_HYPERCALL_MMIO_MAX_DATA_LENGTH];
 } __packed;
 
+/*
+ * VTL call/return hypercall page offsets register
+ */
+union hv_register_vsm_code_page_offsets {
+	u64 as_u64;
+	struct {
+		u64 vtl_call_offset:12;
+		u64 vtl_return_offset:12;
+		u64 reserved:40;
+	} __packed;
+};
 #endif
-- 
2.40.1