Re: [PATCH v2 08/12] x86/sev: Park APs on AP Jump Table with GHCB protocol version 2

From: Joerg Roedel <redacted>

GHCB protocol version 2 adds the MSR-based AP-reset-hold VMGEXIT which
does not need a GHCB. Use that to park APs in 16-bit protected mode on
the AP Jump Table.

Signed-off-by: Joerg Roedel <redacted>
---
 arch/x86/include/asm/realmode.h    |  3 +
 arch/x86/kernel/sev.c              | 48 ++++++++++++++--
 arch/x86/realmode/rm/Makefile      | 11 ++--
 arch/x86/realmode/rm/header.S      |  3 +
 arch/x86/realmode/rm/sev_ap_park.S | 89 ++++++++++++++++++++++++++++++
 5 files changed, 144 insertions(+), 10 deletions(-)
 create mode 100644 arch/x86/realmode/rm/sev_ap_park.S

diff --git a/arch/x86/include/asm/realmode.h b/arch/x86/include/asm/realmode.h
index 29590a4ddf24..668de0a8b1ae 100644
--- a/arch/x86/include/asm/realmode.h
+++ b/arch/x86/include/asm/realmode.h

@@ -23,6 +23,9 @@ struct real_mode_header {
 	u32	trampoline_header;
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 	u32	sev_es_trampoline_start;
+	u32	sev_real_ap_park_asm;

 #endif
 #ifdef CONFIG_X86_64
 	u32	trampoline_pgd;

diff --git a/arch/x86/kernel/sev.c b/arch/x86/kernel/sev.c
index a98eab926682..20b439986d86 100644
--- a/arch/x86/kernel/sev.c
+++ b/arch/x86/kernel/sev.c

@@ -27,6 +27,7 @@
 #include <asm/fpu/internal.h>
 #include <asm/processor.h>
 #include <asm/realmode.h>
+#include <asm/tlbflush.h>
 #include <asm/traps.h>
 #include <asm/svm.h>
 #include <asm/smp.h>

@@ -695,6 +696,35 @@ static bool __init sev_es_setup_ghcb(void)
 }
 
 #ifdef CONFIG_HOTPLUG_CPU
+void __noreturn sev_jumptable_ap_park(void)
+{
+	local_irq_disable();
+
+	write_cr3(real_mode_header->trampoline_pgd);
+
+	/* Exiting long mode will fail if CR4.PCIDE is set. */
+	if (boot_cpu_has(X86_FEATURE_PCID))

+		cr4_clear_bits(X86_CR4_PCIDE);
+
+	asm volatile("xorq	%%r15, %%r15\n"
+		     "xorq	%%r14, %%r14\n"
+		     "xorq	%%r13, %%r13\n"
+		     "xorq	%%r12, %%r12\n"
+		     "xorq	%%r11, %%r11\n"
+		     "xorq	%%r10, %%r10\n"
+		     "xorq	%%r9,  %%r9\n"
+		     "xorq	%%r8,  %%r8\n"
+		     "xorq	%%rsi, %%rsi\n"
+		     "xorq	%%rdi, %%rdi\n"
+		     "xorq	%%rsp, %%rsp\n"
+		     "xorq	%%rbp, %%rbp\n"

+		     "ljmpl	*%0" : :
+		     "m" (real_mode_header->sev_real_ap_park_asm),
+		     "b" (sev_es_jump_table_pa >> 4));

diff --git a/arch/x86/realmode/rm/header.S b/arch/x86/realmode/rm/header.S
index 8c1db5bf5d78..6c17f8fd1eb4 100644
--- a/arch/x86/realmode/rm/header.S
+++ b/arch/x86/realmode/rm/header.S

@@ -22,6 +22,9 @@ SYM_DATA_START(real_mode_header)
 	.long	pa_trampoline_header
 #ifdef CONFIG_AMD_MEM_ENCRYPT
 	.long	pa_sev_es_trampoline_start
+	.long	pa_sev_ap_park_asm
+	.long	__KERNEL32_CS
+	.long	pa_sev_ap_park_gdt;
 #endif
 #ifdef CONFIG_X86_64
 	.long	pa_trampoline_pgd;

diff --git a/arch/x86/realmode/rm/sev_ap_park.S b/arch/x86/realmode/rm/sev_ap_park.S

new file mode 100644
index 000000000000..0b63d0569d4d

--- /dev/null
+++ b/arch/x86/realmode/rm/sev_ap_park.S

@@ -0,0 +1,89 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#include <linux/linkage.h>
+#include <asm/segment.h>
+#include <asm/page_types.h>
+#include <asm/processor-flags.h>
+#include <asm/msr-index.h>
+#include <asm/sev-ap-jumptable.h>
+#include "realmode.h"
+
+	.section ".text32", "ax"
+	.code32
+/*

+ * The following code switches to 16-bit protected mode and sets up the
+ * execution environment for the AP Jump Table blob. Then it jumps to the AP
+ * Jump Table to park the AP.
+ *
+ * The code was copied from reboot.S and modified to fit the SEV-ES requirements
+ * for AP parking.

+ * in reset state.
+ *
+ * The AP Jump Table physical base address is in %EBX upon entry.
+ *
+ * %EAX, %ECX, %EDX and EFLAGS are undefined. Only use registers %EAX-%EDX and
+ * %ESP in this code.
+ */
+SYM_CODE_START(sev_ap_park_asm)

+
+	/* Switch to trampoline GDT as it is guaranteed < 4 GiB */
+	movl	$__KERNEL_DS, %eax
+	movl	%eax, %ds
+	lgdt	pa_tr_gdt
+
+	/* Disable paging to drop us out of long mode */
+	movl	%cr0, %eax
+	btcl	$X86_CR0_PG_BIT, %eax
+	movl	%eax, %cr0
+

+	/* Clear EFER */
+	movl	$0, %eax
+	movl	$0, %edx

+	movl	$MSR_EFER, %ecx
+	wrmsr
+
+	/* Clear CR3 */
+	movl	$0, %ecx

+	movl	%ecx, %cr3
+
+	/* Set up the IDT for real mode. */
+	lidtl	pa_machine_real_restart_idt
+
+	/*
+	 * Load the GDT with the 16-bit segments for the AP Jump Table
+	 */

+	lgdtl	pa_sev_ap_park_gdt
+
+	/* Setup Code and Data segments for AP Jump Table */