Received-SPF: pass (google.com: domain of linux-kernel+bounces-125123-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:4601:e00::3 as permitted sender) client-ip=2604:1380:4601:e00::3;
Message-ID: <b94c81ef-50f2-4e66-9533-461791777d10@intel.com>
Date: Fri, 29 Mar 2024 23:21:32 +0800
Precedence: bulk
MIME-Version: 1.0
User-Agent: Mozilla Thunderbird
Subject: Re: [PATCHv9 05/17] x86/kexec: Keep CR4.MCE set during kexec for TDX
 guest
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
 Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
 Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>,
 x86@kernel.org
Cc: "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>, Baoquan He <bhe@redhat.com>,
 kexec@lists.infradead.org, linux-coco@lists.linux.dev,
 linux-kernel@vger.kernel.org
References: <20240325103911.2651793-1-kirill.shutemov@linux.intel.com>
 <20240325103911.2651793-6-kirill.shutemov@linux.intel.com>
Content-Language: en-US
From: Xiaoyao Li <xiaoyao.li@intel.com>
In-Reply-To: <20240325103911.2651793-6-kirill.shutemov@linux.intel.com>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 7bit

On 3/25/2024 6:38 PM, Kirill A. Shutemov wrote:
> TDX guests are not allowed to clear CR4.MCE. Attempt to clear it leads
> to #VE.

Will we consider making it more safe and compatible for future to guard 
against X86_FEATURE_MCE as well?

If in the future, MCE becomes configurable for TD guest, then CR4.MCE 
might not be fixed1.

> Use alternatives to keep the flag during kexec for TDX guests.
> 
> The change doesn't affect non-TDX-guest environments.
> 
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Reviewed-by: Kai Huang <kai.huang@intel.com>
> Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
> ---
>   arch/x86/kernel/relocate_kernel_64.S | 5 +++++
>   1 file changed, 5 insertions(+)
> 
> diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
> index 56cab1bb25f5..e144bcf60cbe 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>
> @@ -145,12 +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. Clearing MCE is not allowed
> +	 *    in TDX guests.
>   	 */
>   	movl	$X86_CR4_PAE, %eax
>   	testq	$X86_CR4_LA57, %r13
>   	jz	1f
>   	orl	$X86_CR4_LA57, %eax
>   1:
> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %eax), X86_FEATURE_TDX_GUEST
>   	movq	%rax, %cr4
>   
>   	jmp 1f