linux-kernel - Re: [PATCH 10/13] x86/tdx: Convert shared memory back to private on kexec

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Message-ID: <e0459b50-7e21-7548-8151-9010ee88b0a6@amd.com>
Date:   Thu, 5 Oct 2023 17:01:23 -0500
From:   "Kalra, Ashish" <ashish.kalra@....com>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "Rafael J. Wysocki" <rafael@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Jun Nakajima <jun.nakajima@...el.com>,
        Rick Edgecombe <rick.p.edgecombe@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        kexec@...ts.infradead.org, linux-coco@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/13] x86/tdx: Convert shared memory back to private on
 kexec

On 10/5/2023 4:28 PM, Kirill A. Shutemov wrote:
> On Thu, Oct 05, 2023 at 01:41:38PM -0500, Kalra, Ashish wrote:
>>> +static void unshare_all_memory(bool unmap)
>>> +{
>>> +	unsigned long addr, end;
>>> +	long found = 0, shared;
>>> +
>>> +	/*
>>> +	 * Walk direct mapping and convert all shared memory back to private,
>>> +	 */
>>> +
>>> +	addr = PAGE_OFFSET;
>>> +	end  = PAGE_OFFSET + get_max_mapped();
>>> +
>>> +	while (addr < end) {
>>> +		unsigned long size;
>>> +		unsigned int level;
>>> +		pte_t *pte;
>>> +
>>> +		pte = lookup_address(addr, &level);
>>
>> IIRC, you were earlier walking the direct mapping using
>> walk_page_range_novma(), any particular reason to use lookup_address()
>> instead ?
> 
> walk_page_range_novma() wants mmap lock to be taken, but it is tricky as
> we run here from atomic context in case of crash.
> 
> I considered using trylock to bypass the limitation, but it is a hack.
> 
>>
>>> +		size = page_level_size(level);
>>> +
>>> +		if (pte && pte_decrypted(*pte)) {
>>
>> Additionally need to add check for pte_none() here to handle physical memory
>> holes in direct mapping.
> 
> lookup_address() returns NULL for none entries.
>

Looking at lookup_address_in_pgd(), at pte level it is simply returning
pte_offset_kernel() and there does not seem to be a check for returning 
NULL if pte_none() ?

>>> +			int pages = size / PAGE_SIZE;
>>> +
>>> +			/*
>>> +			 * Touching memory with shared bit set triggers implicit
>>> +			 * conversion to shared.
>>> +			 *
>>> +			 * Make sure nobody touches the shared range from
>>> +			 * now on.
>>> +			 *
>>> +			 * Bypass unmapping for crash scenario. Unmapping
>>> +			 * requires sleepable context, but in crash case kernel
>>> +			 * hits the code path with interrupts disabled.
>>
>> In case of SNP we will need to temporarily enable interrupts during this
>> unsharing as we invoke set_memory_encrypted() which then hits a BUG_ON() in
>> cpa_flush() if interrupts are disabled.
> 
> Do you really need full set_memory_encrypted()? Can't you do something
> ligher?
> 
We need to modify the PTE for setting c-bit to 1 so that will require 
cpa_flush(), though probably can add something lighter to do
clflush_cache_range() directly ?

Thanks,
Ashish