linux-kernel - Re: [PATCHv2 18/29] x86/boot: Avoid #VE during boot for TDX platforms

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Message-ID: <20220211161338.uxjgib6psbpjw6t6@black.fi.intel.com>
Date:   Fri, 11 Feb 2022 19:13:38 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...el.com,
        luto@...nel.org, peterz@...radead.org,
        sathyanarayanan.kuppuswamy@...ux.intel.com, aarcange@...hat.com,
        ak@...ux.intel.com, dan.j.williams@...el.com, david@...hat.com,
        hpa@...or.com, jgross@...e.com, jmattson@...gle.com,
        joro@...tes.org, jpoimboe@...hat.com, knsathya@...nel.org,
        pbonzini@...hat.com, sdeep@...are.com, seanjc@...gle.com,
        tony.luck@...el.com, vkuznets@...hat.com, wanpengli@...cent.com,
        x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 18/29] x86/boot: Avoid #VE during boot for TDX platforms

On Wed, Feb 02, 2022 at 01:04:34AM +0100, Thomas Gleixner wrote:
> > +	orl	$X86_CR4_PAE, %eax
> >  	testl	%edx, %edx
> >  	jz	1f
> >  	orl	$X86_CR4_LA57, %eax
> > @@ -662,8 +675,12 @@ SYM_CODE_START(trampoline_32bit_src)
> >  	pushl	$__KERNEL_CS
> >  	pushl	%eax
> >  
> > -	/* Enable paging again */
> > -	movl	$(X86_CR0_PG | X86_CR0_PE), %eax
> > +	/*
> > +	 * Enable paging again.  Keep CR0.NE set, FERR# is no longer used
> > +	 * to handle x87 FPU errors and clearing NE may fault in some
> > +	 * environments.
> 
> FERR# is no longer used is really not informative here. The point is
> that any x86 CPU which is supported by the kernel requires CR0_NE to be
> set. This code was wrong from the very beginning because 64bit CPUs
> never supported #FERR. The reason why it exists is Copy&Pasta without
> brain applied and the sad fact that the hardware does not enforce it in
> native mode for whatever reason. So this want's to be a seperate patch
> with a coherent comment and changelong.

What about the patch below?

Instead of adding CR0.NE there I used CR0_STATE instead or keep existing
value, only modifing required bit.

I'm not familiar with float-point execption handling. I tried to read up
on that in attempt to make coherent commit message. Please correct me if I
wrote something wrong.

---------------------------------8<----------------------------------------

From: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Date: Fri, 11 Feb 2022 14:25:10 +0300
Subject: [PATCH] x86/boot: Set CR0.NE early and keep it set during the boot
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

TDX guest requires CR0.NE to be set. Clearing the bit triggers #GP(0).

If CR0.NE is 0, the MS-DOS compatibility mode for handling floating-point
exceptions is selected. In this mode, the software exception handler for
floating-point exceptions is invoked externally using the processor’s
FERR#, INTR, and IGNNE# pins.

Using FERR# and IGNNE# to handle floating-point exception is deprecated.
CR0.NE=0 also limits newer processors to operate with one logical
processor active.

Kernel uses CR0_STATE constant to initialize CR0. It has NE bit set.
But during early boot has more ad-hoc approach to setting bit in the
register.

Make CR0 initialization consistent, deriving the initial from CR0_STATE.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
---
 arch/x86/boot/compressed/head_64.S   | 7 ++++---
 arch/x86/realmode/rm/trampoline_64.S | 8 ++++----
 2 files changed, 8 insertions(+), 7 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index fd9441f40457..d0c3d33f3542 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -289,7 +289,7 @@ SYM_FUNC_START(startup_32)
 	pushl	%eax
 
 	/* Enter paged protected Mode, activating Long Mode */
-	movl	$(X86_CR0_PG | X86_CR0_PE), %eax /* Enable Paging and Protected mode */
+	movl	$CR0_STATE, %eax
 	movl	%eax, %cr0
 
 	/* Jump from 32bit compatibility mode into 64bit mode. */
@@ -662,8 +662,9 @@ SYM_CODE_START(trampoline_32bit_src)
 	pushl	$__KERNEL_CS
 	pushl	%eax
 
-	/* Enable paging again */
-	movl	$(X86_CR0_PG | X86_CR0_PE), %eax
+	/* Enable paging again. */
+	movl	%cr0, %eax
+	btsl	$X86_CR0_PG_BIT, %eax
 	movl	%eax, %cr0
 
 	lret
diff --git a/arch/x86/realmode/rm/trampoline_64.S b/arch/x86/realmode/rm/trampoline_64.S
index ae112a91592f..d380f2d1fd23 100644
--- a/arch/x86/realmode/rm/trampoline_64.S
+++ b/arch/x86/realmode/rm/trampoline_64.S
@@ -70,7 +70,7 @@ SYM_CODE_START(trampoline_start)
 	movw	$__KERNEL_DS, %dx	# Data segment descriptor
 
 	# Enable protected mode
-	movl	$X86_CR0_PE, %eax	# protected mode (PE) bit
+	movl	$(CR0_STATE & ~X86_CR0_PG), %eax
 	movl	%eax, %cr0		# into protected mode
 
 	# flush prefetch and jump to startup_32
@@ -148,8 +148,8 @@ SYM_CODE_START(startup_32)
 	movl	$MSR_EFER, %ecx
 	wrmsr
 
-	# Enable paging and in turn activate Long Mode
-	movl	$(X86_CR0_PG | X86_CR0_WP | X86_CR0_PE), %eax
+	# Enable paging and in turn activate Long Mode.
+	movl	$CR0_STATE, %eax
 	movl	%eax, %cr0
 
 	/*
@@ -169,7 +169,7 @@ SYM_CODE_START(pa_trampoline_compat)
 	movl	$rm_stack_end, %esp
 	movw	$__KERNEL_DS, %dx
 
-	movl	$X86_CR0_PE, %eax
+	movl	$(CR0_STATE & ~X86_CR0_PG), %eax
 	movl	%eax, %cr0
 	ljmpl   $__KERNEL32_CS, $pa_startup_32
 SYM_CODE_END(pa_trampoline_compat)
-- 
 Kirill A. Shutemov