x86/tdx: Add MSR support for TDX guests

Use hypercall to emulate MSR read/write for the TDX platform.

There are two viable approaches for doing MSRs in a TD guest:

1. Execute the RDMSR/WRMSR instructions like most VMs and bare metal
   do. Some will succeed, others will cause a #VE. All of those that
   cause a #VE will be handled with a TDCALL.
2. Use paravirt infrastructure.  The paravirt hook has to keep a list
   of which MSRs would cause a #VE and use a TDCALL.  All other MSRs
   execute RDMSR/WRMSR instructions directly.

The second option can be ruled out because the list of MSRs was
challenging to maintain. That leaves option #1 as the only viable
solution for the minimal TDX support.

Kernel relies on the exception fixup machinery to handle MSR access
errors. #VE handler uses the same exception fixup code as #GP. It
covers MSR accesses along with other types of fixups.

For performance-critical MSR writes (like TSC_DEADLINE), future patches
will replace the WRMSR/#VE sequence with the direct TDCALL.

RDMSR and WRMSR specification details can be found in
Guest-Host-Communication Interface (GHCI) for Intel Trust Domain
Extensions (Intel TDX) specification, sec titled "TDG.VP.
VMCALL<Instruction.RDMSR>" and "TDG.VP.VMCALL<Instruction.WRMSR>".

Co-developed-by: Kuppuswamy Sathyanarayanan <[email protected]>
Signed-off-by: Kuppuswamy Sathyanarayanan <[email protected]>
Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Andi Kleen <[email protected]>
Reviewed-by: Tony Luck <[email protected]>
Reviewed-by: Dave Hansen <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

arch/x86/coco/tdx/tdx.c

@@ -142,6 +142,44 @@ void __cpuidle tdx_safe_halt(void)
 		WARN_ONCE(1, "HLT instruction emulation failed\n");
 }
 
+static bool read_msr(struct pt_regs *regs)
+{
+	struct tdx_hypercall_args args = {
+		.r10 = TDX_HYPERCALL_STANDARD,
+		.r11 = hcall_func(EXIT_REASON_MSR_READ),
+		.r12 = regs->cx,
+	};
+
+	/*
+	 * Emulate the MSR read via hypercall. More info about ABI
+	 * can be found in TDX Guest-Host-Communication Interface
+	 * (GHCI), section titled "TDG.VP.VMCALL<Instruction.RDMSR>".
+	 */
+	if (__tdx_hypercall(&args, TDX_HCALL_HAS_OUTPUT))
+		return false;
+
+	regs->ax = lower_32_bits(args.r11);
+	regs->dx = upper_32_bits(args.r11);
+	return true;
+}
+
+static bool write_msr(struct pt_regs *regs)
+{
+	struct tdx_hypercall_args args = {
+		.r10 = TDX_HYPERCALL_STANDARD,
+		.r11 = hcall_func(EXIT_REASON_MSR_WRITE),
+		.r12 = regs->cx,
+		.r13 = (u64)regs->dx << 32 | regs->ax,
+	};
+
+	/*
+	 * Emulate the MSR write via hypercall. More info about ABI
+	 * can be found in TDX Guest-Host-Communication Interface
+	 * (GHCI) section titled "TDG.VP.VMCALL<Instruction.WRMSR>".
+	 */
+	return !__tdx_hypercall(&args, 0);
+}
+
 void tdx_get_ve_info(struct ve_info *ve)
 {
 	struct tdx_module_output out;
@@ -178,6 +216,10 @@ static bool virt_exception_kernel(struct pt_regs *regs, struct ve_info *ve)
 	switch (ve->exit_reason) {
 	case EXIT_REASON_HLT:
 		return handle_halt();
+	case EXIT_REASON_MSR_READ:
+		return read_msr(regs);
+	case EXIT_REASON_MSR_WRITE:
+		return write_msr(regs);
 	default:
 		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
 		return false;