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

[Kirill A. Shutemov]

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