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Re: [Qemu-devel] [01/15] postcopy: Transmit and compare individual page

From: Alexey Perevalov
Subject: Re: [Qemu-devel] [01/15] postcopy: Transmit and compare individual page sizes
Date: Wed, 25 Jan 2017 11:01:44 +0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.5.1

Hi David,

I checked you whole patch set with Andrea's kernel

It works and really gives sufficient decreasing of the downtime.

I'm newby in qemu and here in the mailing list.

I have some remarks on current patch.

On both client and server side post copy capability should be enabled

migrate_set_capability postcopy-ram on

and serialization/deserialization relies on it.

 So if destination host
doesn't set post copy capability ram_load will skip reading of
remote_page_size and in case of multiple RAMBlocks the next read of len
will be incorrect. Hopefully usually len is 0, but it could be bigger and overrun buffer ;). Maybe it's better to pass post copy capability attribute from host to destination to avoid
such assumption.

On 01/06/2017 09:28 PM, Dave Gilbert wrote:
From: "Dr. David Alan Gilbert" <address@hidden>

When using postcopy with hugepages, we require the source
and destination page sizes for any RAMBlock to match.

Transmit them as part of the RAM information header and
fail if there's a difference.

Signed-off-by: Dr. David Alan Gilbert <address@hidden>
  migration/ram.c | 15 +++++++++++++++
  1 file changed, 15 insertions(+)

diff --git a/migration/ram.c b/migration/ram.c
index a1c8089..39998f5 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -1970,6 +1970,9 @@ static int ram_save_setup(QEMUFile *f, void *opaque)
          qemu_put_byte(f, strlen(block->idstr));
          qemu_put_buffer(f, (uint8_t *)block->idstr, strlen(block->idstr));
          qemu_put_be64(f, block->used_length);
+        if (migrate_postcopy_ram() && block->page_size != qemu_host_page_size) 
+            qemu_put_be64(f, block->page_size);
+        }
@@ -2536,6 +2539,18 @@ static int ram_load(QEMUFile *f, void *opaque, int 
+                    /* For postcopy we need to check hugepage sizes match */
+                    if (migrate_postcopy_ram() &&
+                        block->page_size != qemu_host_page_size) {
+                        uint64_t remote_page_size = qemu_get_be64(f);
+                        if (remote_page_size != block->page_size) {
+                            error_report("Mismatched RAM page size %s "
+                                         "(local) %" PRId64 "!= %" PRId64,
+                                         id, block->page_size,
+                                         remote_page_size);
+                            ret = -EINVAL;
+                        }
+                    }
                      ram_control_load_hook(f, RAM_CONTROL_BLOCK_REG,
                  } else {

Best regards,
Alexey Perevalov

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