[PATCH v8 11/11] x86/tdx: Handle CPUID via #VE

[Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>]

From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>

When running virtualized, the CPUID instruction is handled differently
based on the leaf being accessed.  The behavior depends only on the
leaf and applies equally to both kernel/ring-0 and userspace/ring-3
execution of CPUID. Historically, there are two basic classes:

 * Leaves handled transparently to the guest
 * Leaves handled by the VMM

In a typical guest without TDX, "handled by the VMM" leaves cause a
VMEXIT.  TDX replaces these VMEXITs with a #VE exception in the guest.
The guest typically handles the #VE by making a hypercall to the VMM.

The TDX module specification [1], section titled "CPUID Virtualization"
talks about a few more classes of CPUID handling. But, for the purposes
of this patch, the "handled transparently" CPUID leaves are all lumped
together because the guest handling is the same.

[1] - https://software.intel.com/content/dam/develop/external/us/en/documents/tdx-module-1.0-public-spec-v0.931.pdf

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
---

Changes since v7: 
 * None

Changes since v6:
 * Fixed commit log as per Dave Hansen comments.
 * Added extra comments for _tdx_hypercall() usage.

Changes since v5:
 * Fixed commit log as per review comments.
 * Removed WARN_ON() in tdx_handle_cpuid().
 * Renamed "tdg" prefix with "tdx".

Changes since v4:
 * None

Changes since v3:
 * None

 arch/x86/kernel/tdx.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/arch/x86/kernel/tdx.c b/arch/x86/kernel/tdx.c
index 062ac4720434..11e367228e96 100644
--- a/arch/x86/kernel/tdx.c
+++ b/arch/x86/kernel/tdx.c
@@ -145,6 +145,31 @@ static int tdx_write_msr_safe(unsigned int msr, unsigned int low,
 	return ret ? -EIO : 0;
 }
 
+static u64 tdx_handle_cpuid(struct pt_regs *regs)
+{
+	struct tdx_hypercall_output out = {0};
+	u64 ret;
+
+	/*
+	 * Emulate CPUID instruction via hypercall. More info about
+	 * ABI can be found in TDX Guest-Host-Communication Interface
+	 * (GHCI), section titled "VP.VMCALL<Instruction.CPUID>".
+	 */
+	ret = _tdx_hypercall(EXIT_REASON_CPUID, regs->ax, regs->cx, 0, 0, &out);
+
+	/*
+	 * As per TDX GHCI CPUID ABI, r12-r15 registers contains contents of
+	 * EAX, EBX, ECX, EDX registers after CPUID instruction execution.
+	 * So copy the register contents back to pt_regs.
+	 */
+	regs->ax = out.r12;
+	regs->bx = out.r13;
+	regs->cx = out.r14;
+	regs->dx = out.r15;
+
+	return ret;
+}
+
 unsigned long tdx_get_ve_info(struct ve_info *ve)
 {
 	struct tdx_module_output out = {0};
@@ -188,6 +213,9 @@ int tdx_handle_virtualization_exception(struct pt_regs *regs,
 	case EXIT_REASON_MSR_WRITE:
 		ret = tdx_write_msr_safe(regs->cx, regs->ax, regs->dx);
 		break;
+	case EXIT_REASON_CPUID:
+		ret = tdx_handle_cpuid(regs);
+		break;
 	default:
 		pr_warn("Unexpected #VE: %lld\n", ve->exit_reason);
 		return -EFAULT;
-- 
2.25.1