[PATCHv2 13/29] x86/tdx: Add port I/O emulation

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

TDX hypervisors cannot emulate instructions directly. This includes
port I/O which is normally emulated in the hypervisor. All port I/O
instructions inside TDX trigger the #VE exception in the guest and
would be normally emulated there.

Use a hypercall to emulate port I/O. Extend the
tdx_handle_virt_exception() and add support to handle the #VE due to
port I/O instructions.

String I/O operations are not supported in TDX. Unroll them by declaring
CC_ATTR_GUEST_UNROLL_STRING_IO confidential computing attribute.

Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
---
 arch/x86/kernel/cc_platform.c |  3 +++
 arch/x86/kernel/tdx.c         | 48 +++++++++++++++++++++++++++++++++++
 2 files changed, 51 insertions(+)

diff --git a/arch/x86/kernel/cc_platform.c b/arch/x86/kernel/cc_platform.c
index c72b3919bca9..8da246ab4339 100644
--- a/arch/x86/kernel/cc_platform.c
+++ b/arch/x86/kernel/cc_platform.c
@@ -17,6 +17,9 @@
 
 static bool intel_cc_platform_has(enum cc_attr attr)
 {
+	if (attr == CC_ATTR_GUEST_UNROLL_STRING_IO)
+		return true;
+
 	return false;
 }
 
diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
index 8e630eeb765d..e73af22a4c11 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -13,6 +13,12 @@
 /* TDX module Call Leaf IDs */
 #define TDX_GET_VEINFO			3
 
+/* See Exit Qualification for I/O Instructions in VMX documentation */
+#define VE_IS_IO_IN(exit_qual)		(((exit_qual) & 8) ? 1 : 0)
+#define VE_GET_IO_SIZE(exit_qual)	(((exit_qual) & 7) + 1)
+#define VE_GET_PORT_NUM(exit_qual)	((exit_qual) >> 16)
+#define VE_IS_IO_STRING(exit_qual)	((exit_qual) & 16 ? 1 : 0)
+
 static bool tdx_guest_detected __ro_after_init;
 
 /*
@@ -257,6 +263,45 @@ static int tdx_handle_mmio(struct pt_regs *regs, struct ve_info *ve)
 	return insn.length;
 }
 
+/*
+ * Emulate I/O using hypercall.
+ *
+ * Assumes the IO instruction was using ax, which is enforced
+ * by the standard io.h macros.
+ *
+ * Return True on success or False on failure.
+ */
+static bool tdx_handle_io(struct pt_regs *regs, u32 exit_qual)
+{
+	struct tdx_hypercall_output out;
+	int size, port, ret;
+	u64 mask;
+	bool in;
+
+	if (VE_IS_IO_STRING(exit_qual))
+		return false;
+
+	in   = VE_IS_IO_IN(exit_qual);
+	size = VE_GET_IO_SIZE(exit_qual);
+	port = VE_GET_PORT_NUM(exit_qual);
+	mask = GENMASK(BITS_PER_BYTE * size, 0);
+
+	/*
+	 * Emulate the I/O read/write via hypercall. More info about
+	 * ABI can be found in TDX Guest-Host-Communication Interface
+	 * (GHCI) sec titled "TDG.VP.VMCALL<Instruction.IO>".
+	 */
+	ret = _tdx_hypercall(EXIT_REASON_IO_INSTRUCTION, size, !in, port,
+			     in ? 0 : regs->ax, &out);
+	if (!in)
+		return !ret;
+
+	regs->ax &= ~mask;
+	regs->ax |= ret ? UINT_MAX : out.r11 & mask;
+
+	return !ret;
+}
+
 bool tdx_get_ve_info(struct ve_info *ve)
 {
 	struct tdx_module_output out;
@@ -333,6 +378,9 @@ static bool tdx_virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
 		if (!ret)
 			pr_warn_once("MMIO failed\n");
 		break;
+	case EXIT_REASON_IO_INSTRUCTION:
+		ret = tdx_handle_io(regs, ve->exit_qual);
+		break;
 	default:
 		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
 		break;
-- 
2.34.1