+/*
+ * ram_block_populate_pages: populate memory in the RAM block by reading
+ * an integer from the beginning of each page.
+ *
+ * Since it's solely used for userfault_fd WP feature, here we just
+ * hardcode page size to TARGET_PAGE_SIZE.
+ *
+ * @bs: RAM block to populate
+ */
+volatile int ram_block_populate_pages__tmp;
+static void ram_block_populate_pages(RAMBlock *bs)
+{
+ ram_addr_t offset = 0;
+ int tmp = 0;
+
+ for (char *ptr = (char *) bs->host; offset < bs->used_length;
+ ptr += TARGET_PAGE_SIZE, offset += TARGET_PAGE_SIZE) {
You'll want qemu_real_host_page_size instead of TARGET_PAGE_SIZE
+ /* Try to do it without memory writes */
+ tmp += *(volatile int *) ptr;
+ }
The following is slightly simpler and doesn't rely on volatile semantics [1].
Should work on any arch I guess.
static void ram_block_populate_pages(RAMBlock *bs)
{
char *ptr = (char *) bs->host;
ram_addr_t offset;
for (offset = 0; offset < bs->used_length;
offset += qemu_real_host_page_size) {
char tmp = *(volatile char *)(ptr + offset)
/* Don't optimize the read out. */
asm volatile ("" : "+r" (tmp));
}
Compiles to
for (offset = 0; offset < bs->used_length;
316d: 48 8b 4b 30 mov 0x30(%rbx),%rcx
char *ptr = (char *) bs->host;
3171: 48 8b 73 18 mov 0x18(%rbx),%rsi
for (offset = 0; offset < bs->used_length;
3175: 48 85 c9 test %rcx,%rcx
3178: 74 ce je 3148
<ram_write_tracking_prepare+0x58>
offset += qemu_real_host_page_size) {
317a: 48 8b 05 00 00 00 00 mov 0x0(%rip),%rax # 3181
<ram_write_tracking_prepare+0x91>
3181: 48 8b 38 mov (%rax),%rdi
3184: 0f 1f 40 00 nopl 0x0(%rax)
char tmp = *(volatile char *)(ptr + offset);
3188: 48 8d 04 16 lea (%rsi,%rdx,1),%rax
318c: 0f b6 00 movzbl (%rax),%eax
offset += qemu_real_host_page_size) {
318f: 48 01 fa add %rdi,%rdx
for (offset = 0; offset < bs->used_length;
3192: 48 39 ca cmp %rcx,%rdx
3195: 72 f1 jb 3188
<ram_write_tracking_prepare+0x98>
[1] https://programfan.github.io/blog/2015/04/27/prevent-gcc-optimize-away-code/
I'll send patches soon to take care of virtio-mem via RamDiscardManager -
to skip populating the parts that are supposed to remain discarded and not
migrated.
Unfortunately, the RamDiscardManager patches are still stuck waiting for
acks ... and now we're in soft-freeze.
--
Thanks,
David / dhildenb
