[PATCH v14 20/22] KVM: selftests: Implement MMIO WRITE for the TDX VM

[Lisa Wang]

From: Erdem Aktas <erdemaktas@xxxxxxxxxx>

Implement the tdx_mmio_write() to allow TDX VMs to request MMIO
emulation.

Follow the Intel Guest-Hypervisor Communication Interface (GHCI) spec
to the minimum extent that a spec-abiding TDX module will pass the
request to KVM. Skip implementing the #VE handler as described in the
GHCI spec to reduce selftests dependency.

To perform emulated I/O, VMs use the TDG.VP.VMCALL instruction to
request MMIO.

Signed-off-by: Erdem Aktas <erdemaktas@xxxxxxxxxx>
Co-developed-by: Sagi Shahar <sagis@xxxxxxxxxx>
Signed-off-by: Sagi Shahar <sagis@xxxxxxxxxx>
Co-developed-by: Lisa Wang <wyihan@xxxxxxxxxx>
Signed-off-by: Lisa Wang <wyihan@xxxxxxxxxx>
---
 tools/testing/selftests/kvm/Makefile.kvm          |  1 +
 tools/testing/selftests/kvm/include/x86/tdx/tdx.h | 17 +++++++++++
 tools/testing/selftests/kvm/lib/x86/tdx/tdx.S     | 37 +++++++++++++++++++++++
 3 files changed, 55 insertions(+)

diff --git a/tools/testing/selftests/kvm/Makefile.kvm b/tools/testing/selftests/kvm/Makefile.kvm
index 645d9aac61db..28b33ed8c2e6 100644
--- a/tools/testing/selftests/kvm/Makefile.kvm
+++ b/tools/testing/selftests/kvm/Makefile.kvm
@@ -33,6 +33,7 @@ LIBKVM_x86 += lib/x86/tdx/tdx_util.c
 LIBKVM_x86 += lib/x86/ucall.c
 LIBKVM_x86 += lib/x86/vmx.c
 LIBKVM_x86 += lib/x86/tdx/td_boot.S
+LIBKVM_x86 += lib/x86/tdx/tdx.S
 
 LIBKVM_arm64 += lib/arm64/gic.c
 LIBKVM_arm64 += lib/arm64/gic_v3.c
diff --git a/tools/testing/selftests/kvm/include/x86/tdx/tdx.h b/tools/testing/selftests/kvm/include/x86/tdx/tdx.h
new file mode 100644
index 000000000000..6355a30bb47f
--- /dev/null
+++ b/tools/testing/selftests/kvm/include/x86/tdx/tdx.h
@@ -0,0 +1,17 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef SELFTEST_KVM_TDX_TDX_H
+#define SELFTEST_KVM_TDX_TDX_H
+
+#include <linux/types.h>
+
+#define TDG_VP_VMCALL_VE_REQUEST_MMIO    48
+#define TDVMCALL_MMIO_WRITE		  1
+
+u64 __tdcall(u64 leaf, u64 r12, u64 r13, u64 r14, u64 r15);
+
+static inline u64 tdx_mmio_write(u64 address, u32 size, u64 data_in)
+{
+	return __tdcall(TDG_VP_VMCALL_VE_REQUEST_MMIO, size,
+			TDVMCALL_MMIO_WRITE, address, data_in);
+}
+#endif /* SELFTEST_KVM_TDX_TDX_H */
diff --git a/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S b/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S
new file mode 100644
index 000000000000..dff48a3624a1
--- /dev/null
+++ b/tools/testing/selftests/kvm/lib/x86/tdx/tdx.S
@@ -0,0 +1,37 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+.globl __tdcall
+/*
+ * u64 __tdcall(u64 leaf, u64 r12, u64 r13, u64 r14, u64 r15)
+ */
+__tdcall:
+	push %r12
+	push %r13
+	push %r14
+	push %r15
+
+	/* Map C ABI fast arguments to TDX GHCI payload registers */
+	/* RDI=leaf, RSI=r12, RDX=r13, RCX=r14, R8=r15 */
+
+	mov %rdi, %r11
+	mov %rsi, %r12
+	mov %rdx, %r13
+	mov %rcx, %r14
+	mov %r8,  %r15
+
+	/* TDCALL boilerplate */
+	mov $0, %rax
+	mov $0xFC00, %rcx	/* Expose R10-R15 */
+	mov $0, %r10
+
+	/* tdcall instruction */
+	.byte 0x66, 0x0f, 0x01, 0xcc
+
+	/* Extract status */
+	mov %r10, %rax
+
+	pop %r15
+	pop %r14
+	pop %r13
+	pop %r12
+	ret

-- 
2.55.0.229.g6434b31f56-goog