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Date: Wed, 12 Jun 2024 16:25:41 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
CC: Borislav Petkov <bp@alien8.de>, 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>,
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        linux-coco@lists.linux.dev, linux-kernel@vger.kernel.org
Subject: =?US-ASCII?Q?Re=3A_=5BPATCHv11_05/19=5D_x86/relocate=5Fkern?=
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On June 12, 2024 4:06:07 PM PDT, Andrew Cooper <andrew=2Ecooper3@citrix=2Ec=
om> wrote:
>On 12/06/2024 10:22 am, Kirill A=2E Shutemov wrote:
>> On Tue, Jun 11, 2024 at 11:26:17AM -0700, H=2E Peter Anvin wrote:
>>> On 6/4/24 08:21, Kirill A=2E Shutemov wrote:
>>>>  From b45fe48092abad2612c2bafbb199e4de80c99545 Mon Sep 17 00:00:00 20=
01
>>>> From: "Kirill A=2E Shutemov" <kirill=2Eshutemov@linux=2Eintel=2Ecom>
>>>> Date: Fri, 10 Feb 2023 12:53:11 +0300
>>>> Subject: [PATCHv11=2E1 06/19] x86/kexec: Keep CR4=2EMCE set during ke=
xec for TDX guest
>>>>
>>>> TDX guests run with MCA enabled (CR4=2EMCE=3D1b) from the very start=
=2E 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=2E
>>>>
>>>> Therefore, make sure the CR4=2EMCE setting is preserved over kexec to=
o and
>>>> avoid raising any #VEs=2E
>>>>
>>>> The change doesn't affect non-TDX-guest environments=2E
>>>>
>>>> Signed-off-by: Kirill A=2E Shutemov <kirill=2Eshutemov@linux=2Eintel=
=2Ecom>
>>>> ---
>>>>   arch/x86/kernel/relocate_kernel_64=2ES | 17 ++++++++++-------
>>>>   1 file changed, 10 insertions(+), 7 deletions(-)
>>>>
>>>> diff --git a/arch/x86/kernel/relocate_kernel_64=2ES b/arch/x86/kernel=
/relocate_kernel_64=2ES
>>>> index 085eef5c3904=2E=2E9c2cf70c5f54 100644
>>>> --- a/arch/x86/kernel/relocate_kernel_64=2ES
>>>> +++ b/arch/x86/kernel/relocate_kernel_64=2ES
>>>> @@ -5,6 +5,8 @@
>>>>    */
>>>>   #include <linux/linkage=2Eh>
>>>> +#include <linux/stringify=2Eh>
>>>> +#include <asm/alternative=2Eh>
>>>>   #include <asm/page_types=2Eh>
>>>>   #include <asm/kexec=2Eh>
>>>>   #include <asm/processor-flags=2Eh>
>>>> @@ -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 befor=
e=2E
>>>> +	 *    Clearing MCE might not be allowed in TDX guests, depending on=
 setup=2E
>>>> +	 *
>>>> +	 * Use R13 that contains the original CR4 value, read in relocate_k=
ernel()=2E
>>>> +	 * PAE is always set in the original CR4=2E
>>>>   	 */
>>>> -	movl	$X86_CR4_PAE, %eax
>>>> -	testq	$X86_CR4_LA57, %r13
>>>> -	jz	=2ELno_la57
>>>> -	orl	$X86_CR4_LA57, %eax
>>>> -=2ELno_la57:
>>>> -
>>>> -	movq	%rax, %cr4
>>>> +	andl	$(X86_CR4_PAE | X86_CR4_LA57), %r13d
>>>> +	ALTERNATIVE "", __stringify(orl $X86_CR4_MCE, %r13d), X86_FEATURE_T=
DX_GUEST
>>>> +	movq	%r13, %cr4
>>> If this is the case, I don't really see a reason to clear MCE per se a=
s I'm
>>> guessing a machine check here will be fatal anyway? It just changes th=
e
>>> method of death=2E
>> Andrew had a strong opinion on method of death here=2E
>>
>> https://lore=2Ekernel=2Eorg/all/1144340e-dd95-ee3b-dabb-579f9a65b3c7@ci=
trix=2Ecom
>
>Not sure if I intended it to come across that strongly, but given a
>choice, the !CR4=2EMCE death is cleaner because at least you're not
>interpreting garbage and trying to use it as a valid IDT=2E
>
>~Andrew

Zorch the IDT if it isn't valid?