Re: [PATCHv11 05/19] x86/relocate_kernel: Use named labels for less confusion

From: "H. Peter Anvin" <hpa@zytor.com>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
	Borislav Petkov <bp@alien8.de>
Cc: Nikolay Borisov <nik.borisov@suse.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>,
	"Kalra, Ashish" <ashish.kalra@amd.com>,
	Sean Christopherson <seanjc@google.com>,
	"Huang, Kai" <kai.huang@intel.com>,
	Ard Biesheuvel <ardb@kernel.org>, Baoquan He <bhe@redhat.com>,
	"K. Y. Srinivasan" <kys@microsoft.com>,
	Haiyang Zhang <haiyangz@microsoft.com>,
	kexec@lists.infradead.org, linux-hyperv@vger.kernel.org,
	linux-acpi@vger.kernel.org, linux-coco@lists.linux.dev,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCHv11 05/19] x86/relocate_kernel: Use named labels for less confusion
Date: Tue, 11 Jun 2024 11:26:17 -0700	[thread overview]
Message-ID: <5c8b3ee9-64c2-4ff3-9cca-ba2672b9635e@zytor.com> (raw)
In-Reply-To: <ehttxqgg7zhbgty5m5uxkduj3xf7soonrzfu4rfw7hccqgdydl@afki66pnree5>

On 6/4/24 08:21, Kirill A. Shutemov wrote:
> 
>  From b45fe48092abad2612c2bafbb199e4de80c99545 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: [PATCHv11.1 06/19] x86/kexec: Keep CR4.MCE set during kexec for TDX guest
> 
> TDX guests run with MCA enabled (CR4.MCE=1b) from the very start. If
> that bit is cleared during CR4 register reprogramming during boot or
> kexec flows, a #VE exception will be raised which the guest kernel
> cannot handle it.
> 
> Therefore, make sure the CR4.MCE setting is preserved over kexec too and
> avoid raising any #VEs.
> 
> The change doesn't affect non-TDX-guest environments.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
>   arch/x86/kernel/relocate_kernel_64.S | 17 ++++++++++-------
>   1 file changed, 10 insertions(+), 7 deletions(-)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 085eef5c3904..9c2cf70c5f54 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>
> @@ -145,14 +147,15 @@ 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 be allowed in TDX guests, depending on setup.
> +	 *
> +	 * Use R13 that contains the original CR4 value, read in relocate_kernel().
> +	 * PAE is always set in the original CR4.
>   	 */
> -	movl	$X86_CR4_PAE, %eax
> -	testq	$X86_CR4_LA57, %r13
> -	jz	.Lno_la57
> -	orl	$X86_CR4_LA57, %eax
> -.Lno_la57:
> -
> -	movq	%rax, %cr4
> +	andl	$(X86_CR4_PAE | X86_CR4_LA57), %r13d
> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %r13d), X86_FEATURE_TDX_GUEST
> +	movq	%r13, %cr4
>   

If this is the case, I don't really see a reason to clear MCE per se as 
I'm guessing a machine check here will be fatal anyway? It just changes 
the method of death.

Also, is there a reason to save %cr4, run code, and *then* clear the 
relevant bits? Wouldn't it be better to sanitize %cr4 as soon as possible?

	-hpa

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