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

[Kirill A. Shutemov]

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.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov