[PATCH v3 2/2] x86/early_printk: Add earlyprintk=tdx to drive the UART with TDVMCALLs

@@ -1543,6 +1543,7 @@ Kernel parameters
 			earlyprintk=serial[,ttySn[,baudrate]]
 			earlyprintk=serial[,0x...[,baudrate]]
 			earlyprintk=ttySn[,baudrate]
+			earlyprintk=tdx
 			earlyprintk=dbgp[debugController#]
 			earlyprintk=mmio32,membase[,{nocfg|baudrate}]
 			earlyprintk=pciserial[,force],bus:device.function[,{nocfg|baudrate}]

@@ -1556,6 +1557,14 @@ Kernel parameters
 			Use "nocfg" to skip UART configuration, assume
 			BIOS/firmware has configured UART correctly.
 
+			On x86, "tdx" is a fixed COM1 console, and takes no
+			options. In a TDX guest the port is driven with
+			TDVMCALLs rather than port I/O instructions, avoiding
+			the #VE exception that would otherwise be taken to
+			emulate every access. Outside a TDX guest it falls back
+			to port I/O, behaving like "earlyprintk=serial". If
+			CONFIG_INTEL_TDX_GUEST is not set, it is ignored.
+
 			Append ",keep" to not disable it when the real console
 			takes over.
 

@@ -83,6 +83,9 @@ int tdx_mcall_extend_rtmr(u8 index, u8 *data);
 
 u64 tdx_hcall_get_quote(u8 *buf, size_t size);
 
+u8 tdx_inb(u16 port);
+void tdx_outb(u8 value, u16 port);
+
 void __init tdx_dump_attributes(u64 td_attr);
 void __init tdx_dump_td_ctls(u64 td_ctls);
 

@@ -194,6 +194,45 @@ u64 tdx_hcall_get_quote(u8 *buf, size_t size)
 }
 EXPORT_SYMBOL_GPL(tdx_hcall_get_quote);
 
+/**
+ * tdx_inb() - Read a byte from an I/O port without a #VE
+ * @port: I/O port to read from
+ *
+ * Ask the VMM to perform the read with TDG.VP.VMCALL<Instruction.IO>, rather
+ * than executing an IN instruction and having the resulting #VE emulate it.
+ *
+ * Return: the byte read, or 0xFF if the hypercall failed.
+ */
+u8 tdx_inb(u16 port)
+{
+	struct tdx_module_args args = {
+		.r10 = TDX_HYPERCALL_STANDARD,
+		.r11 = hcall_func(EXIT_REASON_IO_INSTRUCTION),
+		.r12 = 1,
+		.r13 = TDVMCALL_PORT_READ,
+		.r14 = port,
+	};
+
+	if (__tdx_hypercall(&args))
+		return 0xFF;
+
+	return args.r11;
+}
+
+/**
+ * tdx_outb() - Write a byte to an I/O port without a #VE
+ * @value: byte to write
+ * @port: I/O port to write to
+ *
+ * Ask the VMM to perform the write with TDG.VP.VMCALL<Instruction.IO>, rather
+ * than executing an OUT instruction and having the resulting #VE emulate it.
+ */
+void tdx_outb(u8 value, u16 port)
+{
+	_tdx_hypercall(hcall_func(EXIT_REASON_IO_INSTRUCTION), 1,
+		       TDVMCALL_PORT_WRITE, port, value);
+}
+
 static void __noreturn tdx_panic(const char *msg)
 {
 	struct tdx_module_args args = {

@@ -21,6 +21,7 @@
 #include <linux/usb/xhci-dbgp.h>
 #include <asm/pci_x86.h>
 #include <linux/static_call.h>
+#include <asm/tdx.h>
 
 /* Simple VGA output */
 #define VGABASE		(__ISA_IO_base + 0xb8000)

@@ -111,6 +112,23 @@ ANNOTATE_NOENDBR_SYM(io_serial_out);
 DEFINE_STATIC_CALL(serial_in, io_serial_in);
 DEFINE_STATIC_CALL(serial_out, io_serial_out);
 
+#ifdef CONFIG_INTEL_TDX_GUEST
+/*
+ * A TDX guest cannot execute port I/O instructions, so ask the VMM to do it.
+ */
+static __noendbr unsigned int tdx_serial_in(unsigned long addr, int offset)
+{
+	return tdx_inb(addr + offset);
+}
+ANNOTATE_NOENDBR_SYM(tdx_serial_in);
+
+static __noendbr void tdx_serial_out(unsigned long addr, int offset, int value)
+{
+	tdx_outb(value, addr + offset);
+}
+ANNOTATE_NOENDBR_SYM(tdx_serial_out);
+#endif /* CONFIG_INTEL_TDX_GUEST */
+
 static int early_serial_putc(unsigned char ch)
 {
 	unsigned timeout = 0xffff;

@@ -196,6 +214,22 @@ static __init void early_serial_init(char *s)
 	early_serial_hw_init(divisor);
 }
 
+#ifdef CONFIG_INTEL_TDX_GUEST
+/*
+ * Use early_serial_base (COM1) and same default baud rate as
+ * "earlyprintk=serial"
+ */
+static __init void early_serial_tdx_init(void)
+{
+	if (cpu_feature_enabled(X86_FEATURE_TDX_GUEST)) {
+		static_call_update(serial_in, tdx_serial_in);
+		static_call_update(serial_out, tdx_serial_out);
+	}
+
+	early_serial_hw_init(115200 / DEFAULT_BAUD);
+}
+#endif /* CONFIG_INTEL_TDX_GUEST */
+
 static __noendbr void mem32_serial_out(unsigned long addr, int offset, int value)
 {
 	u32 __iomem *vaddr = (u32 __iomem *)addr;

@@ -414,6 +448,13 @@ static int __init setup_early_printk(char *buf)
 			early_serial_init(buf + 4);
 			early_console_register(&early_serial_console, keep);
 		}
+#ifdef CONFIG_INTEL_TDX_GUEST
+		if (!strncmp(buf, "tdx", 3)) {
+			early_serial_tdx_init();
+			early_console_register(&early_serial_console, keep);
+			break;
+		}
+#endif
 #ifdef CONFIG_PCI
 		if (!strncmp(buf, "pciserial", 9)) {
 			buf += 9; /* Keep from match the above "pciserial" */