linux-kernel - Re: [PATCH 2/2] x86/snp: Convert shared memory back to private on kexec

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Date: Thu, 22 Feb 2024 07:58:21 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc: Ashish Kalra <Ashish.Kalra@....com>, tglx@...utronix.de,
 mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
 luto@...nel.org, x86@...nel.org, ardb@...nel.org, hpa@...or.com,
 linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org, rafael@...nel.org,
 peterz@...radead.org, adrian.hunter@...el.com,
 sathyanarayanan.kuppuswamy@...ux.intel.com, elena.reshetova@...el.com,
 jun.nakajima@...el.com, rick.p.edgecombe@...el.com, seanjc@...gle.com,
 kai.huang@...el.com, bhe@...hat.com, kexec@...ts.infradead.org,
 linux-coco@...ts.linux.dev, anisinha@...hat.com, michael.roth@....com,
 bdas@...hat.com, vkuznets@...hat.com, dionnaglaze@...gle.com,
 jroedel@...e.de, ashwin.kamat@...adcom.com
Subject: Re: [PATCH 2/2] x86/snp: Convert shared memory back to private on
 kexec

On 2/22/24 04:50, Kirill A. Shutemov wrote:
> On Wed, Feb 21, 2024 at 02:35:13PM -0600, Tom Lendacky wrote:
>>> @@ -906,6 +917,206 @@ void snp_accept_memory(phys_addr_t start, phys_addr_t end)
>>>    	set_pages_state(vaddr, npages, SNP_PAGE_STATE_PRIVATE);
>>>    }
>>> +static inline bool pte_decrypted(pte_t pte)
>>> +{
>>> +	return cc_mkdec(pte_val(pte)) == pte_val(pte);
>>> +}
>>> +
>>
>> This is duplicated in TDX code, arch/x86/coco/tdx/tdx.c, looks like
>> something that can go in a header file, maybe mem_encrypt.h.
>>
> 
> I think <asm/pgtable.h> is a better fit.
> 
>>> +void snp_kexec_stop_conversion(bool crash)
>>> +{
>>> +	/* Stop new private<->shared conversions */
>>> +	conversion_allowed = false;
>>> +	crash_requested = crash;
>>> +
>>> +	/*
>>> +	 * Make sure conversion_allowed is cleared before checking
>>> +	 * conversions_in_progress.
>>> +	 */
>>> +	barrier();
>>
>> This should be smp_wmb().
>>
> 
> Why?

IIUC, this is because conversions_in_progress can be set on another thread 
and so this needs an smp barrier. In this case, smp_wmb() just ends up 
being barrier(), but to me it is clearer this way. Just my opinion, though.

Thanks,
Tom

>