On Wed, Feb 03, 2021, Will Deacon wrote:

On Fri, Jan 08, 2021 at 12:15:14PM +0000, Quentin Perret wrote:

...

quoted

+static inline unsigned long hyp_s1_pgtable_size(void)
+{

...

quoted

+		res += nr_pages << PAGE_SHIFT;
+	}
+
+	/* Allow 1 GiB for private mappings */
+	nr_pages = (1 << 30) >> PAGE_SHIFT;

SZ_1G >> PAGE_SHIFT

Where does the 1gb magic number come from?  IIUC, this is calculating the number
of pages needed for the hypervisor's Stage-1 page tables.  The amount of memory
needed for those page tables should be easily calculated, and assuming huge
pages can be used, should be far less the 1gb.
 

quoted

+	nr_pages = __hyp_pgtable_max_pages(nr_pages);
+	res += nr_pages << PAGE_SHIFT;
+
+	return res;

...

quoted

+void __init kvm_hyp_reserve(void)
+{
+	u64 nr_pages, prev;
+
+	if (!is_hyp_mode_available() || is_kernel_in_hyp_mode())
+		return;
+
+	if (kvm_get_mode() != KVM_MODE_PROTECTED)
+		return;
+
+	if (kvm_nvhe_sym(hyp_memblock_nr) < 0) {
+		kvm_err("Failed to register hyp memblocks\n");
+		return;
+	}
+
+	sort_memblock_regions();
+
+	/*
+	 * We don't know the number of possible CPUs yet, so allocate for the
+	 * worst case.
+	 */
+	hyp_mem_size += NR_CPUS << PAGE_SHIFT;

Is this for per-cpu stack?

If so, what guarantees a single page is sufficient?  Mostly a curiosity question,
since it looks like this is an existing assumption by init_hyp_mode().  Shouldn't
the required stack size be defined in bytes and converted to pages, or is there a
guarantee that 64kb pages will be used?

There was a recent patch bumping NR_CPUs to 512, so this would be 32MB
with 64k pages. Is it possible to return memory to the host later on once
we have a better handle on the number of CPUs in the system?

Does kvm_hyp_reserve() really need to be called during bootmem_init()?  What
prevents doing the reservation during init_hyp_mode()?  If the problem is that
pKVM needs a single contiguous chunk of memory, then it might be worth solving
_that_ problem, e.g. letting the host donate memory in N-byte chunks instead of
requiring a single huge blob of memory.
 

quoted

+	hyp_mem_size += hyp_s1_pgtable_size();

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