Messages in this thread | | | Date | Fri, 6 Oct 2023 01:28:39 +0300 | From | "Kirill A. Shutemov" <> | Subject | Re: [PATCH 10/13] x86/tdx: Convert shared memory back to private on kexec |
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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.
> > > > + 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 ?
For TDX, I don't touch shared bit as nobody suppose to touch the memory after the point (ans set_memory_np() enforces it for !crash case).
Can't SNP do the same?
-- Kiryl Shutsemau / Kirill A. Shutemov
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