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

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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() ?

+			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 ?

Thanks,
Ashish


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