linux-kernel - Re: [PATCH v6 2/5] x86/kexec: do unconditional WBINVD for bare-metal in relocate

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Message-ID: <3f419ba5-7b95-45f0-adb3-d7397949c14e@intel.com>
Date: Tue, 10 Sep 2024 14:42:20 +1200
From: "Huang, Kai" <kai.huang@...el.com>
To: "Kaplan, David" <David.Kaplan@....com>, "Hansen, Dave"
	<dave.hansen@...el.com>, "bp@...en8.de" <bp@...en8.de>, "tglx@...utronix.de"
	<tglx@...utronix.de>, "peterz@...radead.org" <peterz@...radead.org>,
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	"kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
CC: "x86@...nel.org" <x86@...nel.org>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "pbonzini@...hat.com" <pbonzini@...hat.com>,
	"seanjc@...gle.com" <seanjc@...gle.com>, "Williams, Dan J"
	<dan.j.williams@...el.com>, "Lendacky, Thomas" <Thomas.Lendacky@....com>,
	"Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "Yamahata, Isaku"
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	<nik.borisov@...e.com>, "sagis@...gle.com" <sagis@...gle.com>, Dave Young
	<dyoung@...hat.com>
Subject: Re: [PATCH v6 2/5] x86/kexec: do unconditional WBINVD for bare-metal
 in relocate_kernel()


>> --- a/arch/x86/kernel/machine_kexec_64.c
>> +++ b/arch/x86/kernel/machine_kexec_64.c
>> @@ -322,16 +322,9 @@ void machine_kexec_cleanup(struct kimage *image)
>> void machine_kexec(struct kimage *image)  {
>>          unsigned long page_list[PAGES_NR];
>> -       unsigned int host_mem_enc_active;
>>          int save_ftrace_enabled;
>>          void *control_page;
>>
>> -       /*
>> -        * This must be done before load_segments() since if call depth tracking
>> -        * is used then GS must be valid to make any function calls.
>> -        */
>> -       host_mem_enc_active =
>> cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT);
>> -
> 
> Functionally the patch looks fine.  I would suggest keeping some form of this comment though, because the limitation about not being able to make function calls after load_segments() is arguably non-obvious and this comment served as a warning for future modifications in this area.

Yeah this makes sense.  Thanks.

I think we can add some text to the existing comment of load_segments() 
to call out this.  Allow me to dig into more about call depth tracking 
to understand it better -- relocate_kernel() after load_segments() seems 
to be a real function call and I want to know how does it interact with 
call depth tracking.