qemu-devel
[Top][All Lists]
Advanced

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [PATCH 3/3] migration: Pre-fault memory before starting background s


From: David Hildenbrand
Subject: Re: [PATCH 3/3] migration: Pre-fault memory before starting background snasphot
Date: Fri, 19 Mar 2021 10:28:52 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.0

+/*
+ * 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




reply via email to

[Prev in Thread] Current Thread [Next in Thread]