[RFC v2 23/32] x86/boot: Avoid unnecessary #VE during boot process

[Kuppuswamy Sathyanarayanan]

From: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>

Skip writing EFER during secondary_startup_64() if the current value is
also the desired value. This avoids a #VE when running as a TDX guest,
as the TDX-Module does not allow writes to EFER (even when writing the
current, fixed value).

Also, preserve CR4.MCE instead of clearing it during boot to avoid a #VE
when running as a TDX guest. The TDX-Module (effectively part of the
hypervisor) requires CR4.MCE to be set at all times and injects a #VE
if the guest attempts to clear CR4.MCE.

Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>
---
 arch/x86/boot/compressed/head_64.S |  5 ++++-
 arch/x86/kernel/head_64.S          | 13 +++++++++++--
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index 37c2f37d4a0d..2d79e5f97360 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -622,7 +622,10 @@ SYM_CODE_START(trampoline_32bit_src)
 	popl	%ecx
 
 	/* Enable PAE and LA57 (if required) paging modes */
-	movl	$X86_CR4_PAE, %eax
+	movl	%cr4, %eax
+	/* Clearing CR4.MCE will #VE on TDX guests.  Leave it alone. */
+	andl	$X86_CR4_MCE, %eax
+	orl	$X86_CR4_PAE, %eax
 	testl	%edx, %edx
 	jz	1f
 	orl	$X86_CR4_LA57, %eax
diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 04bddaaba8e2..92c77cf75542 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -141,7 +141,10 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 1:
 
 	/* Enable PAE mode, PGE and LA57 */
-	movl	$(X86_CR4_PAE | X86_CR4_PGE), %ecx
+	movq	%cr4, %rcx
+	/* Clearing CR4.MCE will #VE on TDX guests.  Leave it alone. */
+	andl	$X86_CR4_MCE, %ecx
+	orl	$(X86_CR4_PAE | X86_CR4_PGE), %ecx
 #ifdef CONFIG_X86_5LEVEL
 	testl	$1, __pgtable_l5_enabled(%rip)
 	jz	1f
@@ -229,13 +232,19 @@ SYM_INNER_LABEL(secondary_startup_64_no_verify, SYM_L_GLOBAL)
 	/* Setup EFER (Extended Feature Enable Register) */
 	movl	$MSR_EFER, %ecx
 	rdmsr
+	movl    %eax, %edx
 	btsl	$_EFER_SCE, %eax	/* Enable System Call */
 	btl	$20,%edi		/* No Execute supported? */
 	jnc     1f
 	btsl	$_EFER_NX, %eax
 	btsq	$_PAGE_BIT_NX,early_pmd_flags(%rip)
-1:	wrmsr				/* Make changes effective */
 
+	/* Skip the WRMSR if the current value matches the desired value. */
+1:	cmpl	%edx, %eax
+	je	1f
+	xor	%edx, %edx
+	wrmsr				/* Make changes effective */
+1:
 	/* Setup cr0 */
 	movl	$CR0_STATE, %eax
 	/* Make changes effective */
-- 
2.25.1