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

[Kiryl Shutsemau <kas@kernel.org>]

On Mon, Sep 28, 2026 at 11:07:52PM +0000, Verma, Vishal L wrote:
> On Fri, 2026-09-25 at 12:30 +0100, Kiryl Shutsemau wrote:
> 
> > 
> > All these #ifdefs in early_printk.c can go. X86_FEATURE_TDX_GUEST is a
> > disabled feature without CONFIG_INTEL_TDX_GUEST, so
> > cpu_feature_enabled() folds to zero and the compiler drops the
> > static_call_update() calls and the wrappers with them.
> > 
> > Add stubs for tdx_inb() and tdx_outb() to the #else branch in
> > <asm/tdx.h> and make the "tdx" case unconditional.
> 
> Tried this - it causes an objtool error:
> 
>      CC      arch/x86/kernel/early_printk.o
>    arch/x86/kernel/early_printk.o: error: objtool: bad .discard.annotate_insn entry: 4 of type 1
>    make: *** [Makefile:248: __sub-make] Error 2
> 
> It can be fixed by adding a __used annotation to the wrappers, but is that
> ok?
> 

ANNOTATE_NOENDBR_SYM() is a top-level asm() that references the symbol.
Once the compiler drops the wrapper, the annotation points at an
undefined symbol and objtool flags it.

__used makes it go away by keeping the dead wrappers in every non-TDX
kernel, which is the opposite of what dropping the #ifdefs was for.

Drop the wrappers instead.

Let tdx.c provide the accessors with the serial_in/serial_out signature,
__noendbr and annotated there, and declare them outside the #ifdef. No
stubs are needed: the only reference is under cpu_feature_enabled(),
which folds to zero without CONFIG_INTEL_TDX_GUEST.

Builds with TDX on and off.

diff --git a/arch/x86/coco/tdx/tdx.c b/arch/x86/coco/tdx/tdx.c
index d8887a2a524b..c5e16ea3844d 100644
--- a/arch/x86/coco/tdx/tdx.c
+++ b/arch/x86/coco/tdx/tdx.c
@@ -194,23 +194,19 @@ 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.
+/*
+ * early_printk accessors for a TDX guest. Ask the VMM to do the port I/O with
+ * TDG.VP.VMCALL<Instruction.IO> instead of executing IN/OUT and having the
+ * resulting #VE emulate it.
  */
-u8 tdx_inb(u16 port)
+__noendbr unsigned int tdx_serial_in(unsigned long addr, int offset)
 {
 	struct tdx_module_args args = {
 		.r10 = TDX_HYPERCALL_STANDARD,
 		.r11 = hcall_func(EXIT_REASON_IO_INSTRUCTION),
 		.r12 = 1,
 		.r13 = TDVMCALL_PORT_READ,
-		.r14 = port,
+		.r14 = addr + offset,
 	};
 
 	if (__tdx_hypercall(&args))
@@ -218,20 +214,14 @@ u8 tdx_inb(u16 port)
 
 	return args.r11;
 }
+ANNOTATE_NOENDBR_SYM(tdx_serial_in);
 
-/**
- * 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)
+__noendbr void tdx_serial_out(unsigned long addr, int offset, int value)
 {
 	_tdx_hypercall(hcall_func(EXIT_REASON_IO_INSTRUCTION), 1,
-		       TDVMCALL_PORT_WRITE, port, value);
+		       TDVMCALL_PORT_WRITE, addr + offset, value);
 }
+ANNOTATE_NOENDBR_SYM(tdx_serial_out);
 
 static void __noreturn tdx_panic(const char *msg)
 {
diff --git a/arch/x86/include/asm/tdx.h b/arch/x86/include/asm/tdx.h
index 325dd7c5929f..0bbabcf0791e 100644
--- a/arch/x86/include/asm/tdx.h
+++ b/arch/x86/include/asm/tdx.h
@@ -83,9 +83,6 @@ 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);
 
@@ -98,6 +95,10 @@ static inline bool tdx_early_handle_ve(struct pt_regs *regs) { return false; }
 
 #endif /* CONFIG_INTEL_TDX_GUEST */
 
+/* Only referenced when X86_FEATURE_TDX_GUEST is enabled, no stubs needed */
+unsigned int tdx_serial_in(unsigned long addr, int offset);
+void tdx_serial_out(unsigned long addr, int offset, int value);
+
 #if defined(CONFIG_KVM_GUEST) && defined(CONFIG_INTEL_TDX_GUEST)
 long tdx_kvm_hypercall(unsigned int nr, unsigned long p1, unsigned long p2,
 		       unsigned long p3, unsigned long p4);
diff --git a/arch/x86/kernel/early_printk.c b/arch/x86/kernel/early_printk.c
index 194737a3e9a0..04dadcd73b85 100644
--- a/arch/x86/kernel/early_printk.c
+++ b/arch/x86/kernel/early_printk.c
@@ -112,23 +112,6 @@ 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;
@@ -214,7 +197,6 @@ 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"
@@ -228,7 +210,6 @@ static __init void early_serial_tdx_init(void)
 
 	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)
 {
@@ -448,13 +429,11 @@ 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" */
-- 
  Kiryl Shutsemau / Kirill A. Shutemov