Received-SPF: pass (google.com: domain of linux-kernel+bounces-161515-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Date: Sun, 28 Apr 2024 19:11:11 +0200
From: Borislav Petkov <bp@alien8.de>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Sean Christopherson <seanjc@google.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>,
	Elena Reshetova <elena.reshetova@intel.com>,
	Jun Nakajima <jun.nakajima@intel.com>,
	Rick Edgecombe <rick.p.edgecombe@intel.com>,
	Tom Lendacky <thomas.lendacky@amd.com>,
	Ashish Kalra <ashish.kalra@amd.com>,
	Kai Huang <kai.huang@intel.com>, Baoquan He <bhe@redhat.com>,
	kexec@lists.infradead.org, linux-coco@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCHv10 05/18] x86/kexec: Keep CR4.MCE set during kexec for
 TDX guest
Message-ID: <20240428171111.GKZi6DLy_ZwuZsZdFq@fat_crate.local>
References: <20240409113010.465412-1-kirill.shutemov@linux.intel.com>
 <20240409113010.465412-6-kirill.shutemov@linux.intel.com>
 <ZhVPIDDLkjOB96fI@google.com>
 <3q6jv3g4tezybmd667mqxio7ty22akxv7okrznmzx3tju2u4qo@2alzjkbgm2lh>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <3q6jv3g4tezybmd667mqxio7ty22akxv7okrznmzx3tju2u4qo@2alzjkbgm2lh>

On Tue, Apr 09, 2024 at 06:26:05PM +0300, Kirill A. Shutemov wrote:
> From 6be428e3b1c6fb494b2c48ba6a7c133514a0b2b4 Mon Sep 17 00:00:00 2001
> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
> Date: Fri, 10 Feb 2023 12:53:11 +0300
> Subject: [PATCHv10.1 05/18] x86/kexec: Keep CR4.MCE set during kexec for TDX guest
> 
> Depending on setup, TDX guests might be allowed to clear CR4.MCE.
> Attempt to clear it leads to #VE.
> 
> Use alternatives to keep the flag during kexec for TDX guests.
> 
> The change doesn't affect non-TDX-guest environments.

This is all fine and dandy but nothing explains *why* TDX needs this
special dance.

Why can't TDX do the usual CR4.MCE diddling like the normal kernel
during init and needs to do that here immediately?

> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>  arch/x86/kernel/relocate_kernel_64.S | 15 +++++++++------
>  1 file changed, 9 insertions(+), 6 deletions(-)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 56cab1bb25f5..90246d544eb1 100644
> --- a/arch/x86/kernel/relocate_kernel_64.S
> +++ b/arch/x86/kernel/relocate_kernel_64.S
> @@ -5,6 +5,8 @@
>   */
>  
>  #include <linux/linkage.h>
> +#include <linux/stringify.h>
> +#include <asm/alternative.h>
>  #include <asm/page_types.h>
>  #include <asm/kexec.h>
>  #include <asm/processor-flags.h>
> @@ -143,14 +145,15 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
>  
>  	/*
>  	 * Set cr4 to a known state:
> -	 *  - physical address extension enabled
>  	 *  - 5-level paging, if it was enabled before
> +	 *  - Machine check exception on TDX guest, if it was enabled before.
> +	 *    Clearing MCE might not allowed in TDX guests, depending on setup.

			 ... might not be allowed ...

> +	 *  - physical address extension enabled
>  	 */
> -	movl	$X86_CR4_PAE, %eax
> -	testq	$X86_CR4_LA57, %r13
> -	jz	1f
> -	orl	$X86_CR4_LA57, %eax
> -1:
> +	movl	$X86_CR4_LA57, %eax
> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST
> +	andl	%r13d, %eax

%r13 needs a comment here that it contains %cr4 read above in
relocate_kernel()

> +	orl	$X86_CR4_PAE, %eax
>  	movq	%rax, %cr4
>  
>  	jmp 1f
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette