linux-kernel - Re: [PATCH v3 1/5] x86/kexec: do unconditional WBINVD for bare-metal in stop_this

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Date: Thu, 11 Apr 2024 09:54:13 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
CC: <linux-kernel@...r.kernel.org>, <x86@...nel.org>, <dave.hansen@...el.com>,
	<bp@...en8.de>, <tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>,
	<luto@...nel.org>, <peterz@...radead.org>, <rick.p.edgecombe@...el.com>,
	<thomas.lendacky@....com>, <ashish.kalra@....com>, <chao.gao@...el.com>,
	<bhe@...hat.com>, <nik.borisov@...e.com>, <pbonzini@...hat.com>,
	<seanjc@...gle.com>
Subject: Re: [PATCH v3 1/5] x86/kexec: do unconditional WBINVD for bare-metal
 in stop_this_cpu()



On 11/04/2024 2:12 am, Kirill A. Shutemov wrote:
> On Mon, Apr 08, 2024 at 12:44:54AM +1200, Kai Huang wrote:
>> TL;DR:
> 
> The commit message is waaay too verbose for no good reason. You don't
> really need to repeat all the history around this code.

Could you be more specific?

I was following Boris's suggestion to summerize all the discussion 
around the "unconditional WBINVD" issue.

https://lore.kernel.org/linux-kernel/20240228110207.GCZd8Sr8mXHA2KTiLz@fat_crate.local/

I can try to improve if I can know specifically what should be trimmed down.

> 
>> ---
>>   arch/x86/kernel/process.c | 18 ++++++++----------
>>   1 file changed, 8 insertions(+), 10 deletions(-)
>>
>> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
>> index b8441147eb5e..5ba8a9c1e47a 100644
>> --- a/arch/x86/kernel/process.c
>> +++ b/arch/x86/kernel/process.c
>> @@ -813,18 +813,16 @@ void __noreturn stop_this_cpu(void *dummy)
>>   	mcheck_cpu_clear(c);
>>   
>>   	/*
>> -	 * Use wbinvd on processors that support SME. This provides support
>> -	 * for performing a successful kexec when going from SME inactive
>> -	 * to SME active (or vice-versa). The cache must be cleared so that
>> -	 * if there are entries with the same physical address, both with and
>> -	 * without the encryption bit, they don't race each other when flushed
>> -	 * and potentially end up with the wrong entry being committed to
>> -	 * memory.
>> +	 * The kernel could leave caches in incoherent state on SME/TDX
>> +	 * capable platforms.  Flush cache to avoid silent memory
>> +	 * corruption for these platforms.
>>   	 *
>> -	 * Test the CPUID bit directly because the machine might've cleared
>> -	 * X86_FEATURE_SME due to cmdline options.
>> +	 * stop_this_cpu() is not a fast path, just do unconditional
>> +	 * WBINVD for simplicity.  But only do WBINVD for bare-metal
>> +	 * as TDX guests and SEV-ES/SEV-SNP guests will get unexpected
>> +	 * (and unnecessary) #VE and may unable to handle.
> 
> s/#VE/exception/
> 
> On SEV it is #VC, not #VE.
> 

Thanks.  I think I'll use "exception (#VE or #VC)" which is clearer, as 
Tom typed in the comments to patch 2.