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

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Date:   Fri, 6 Oct 2023 00:28:28 +0300
From:   "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
To:     "Kalra, Ashish" <ashish.kalra@....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 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.

> > +			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?


-- 
  Kiryl Shutsemau / Kirill A. Shutemov