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

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Message-ID: <20231020093952.nx3a6fid2jqdumnw@box>
Date:   Fri, 20 Oct 2023 12:39:52 +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 Fri, Oct 20, 2023 at 12:21:11PM +0300, Kirill A. Shutemov wrote:
> On Fri, Oct 06, 2023 at 02:24:11PM -0500, Kalra, Ashish wrote:
> > 
> > On 10/5/2023 5:28 PM, Kirill A. Shutemov wrote:
> > > On Thu, Oct 05, 2023 at 05:01:23PM -0500, Kalra, Ashish wrote:
> > > > 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() ?
> > > 
> > > Hm. You are right.
> > > 
> > > I think it yet another quirk in how lookup_address() implemented. We need
> > > to make it straight too.
> > > 
> > > There's two options: either make lookup_address() return pointer for entry
> > > even if it is NULL, or add check for pte_none() after pte_offset_kernel()
> > > and return NULL if it is true.
> > > 
> > > I like the first option more as it allows caller to populate the entry if
> > > it wants.
> > 
> > Yes, i like the first option.
> 
> I tried to this, but lookup_address() has to many callers. It gets beyond
> the scope of the patchset. I will add pte_none() check on unshare side for
> now.

Ah. pte_none() is not need for TDX implementation, as pte_decrypted()
check will fail for it. SEV implementation would need an additional check.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov