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Re: [Qemu-devel] [PATCH v4 2/4] migration: API to clear bits of guest fr


From: Wei Wang
Subject: Re: [Qemu-devel] [PATCH v4 2/4] migration: API to clear bits of guest free pages from the dirty bitmap
Date: Thu, 15 Mar 2018 18:52:41 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.7.0

On 03/15/2018 02:11 AM, Dr. David Alan Gilbert wrote:
* Wei Wang (address@hidden) wrote:
This patch adds an API to clear bits corresponding to guest free pages
from the dirty bitmap. Spilt the free page block if it crosses the QEMU
RAMBlock boundary.

Signed-off-by: Wei Wang <address@hidden>
CC: Dr. David Alan Gilbert <address@hidden>
CC: Juan Quintela <address@hidden>
CC: Michael S. Tsirkin <address@hidden>
---
  include/migration/misc.h |  2 ++
  migration/ram.c          | 21 +++++++++++++++++++++
  2 files changed, 23 insertions(+)

diff --git a/include/migration/misc.h b/include/migration/misc.h
index 77fd4f5..fae1acf 100644
--- a/include/migration/misc.h
+++ b/include/migration/misc.h
@@ -14,11 +14,13 @@
  #ifndef MIGRATION_MISC_H
  #define MIGRATION_MISC_H
+#include "exec/cpu-common.h"
  #include "qemu/notify.h"
/* migration/ram.c */ void ram_mig_init(void);
+void qemu_guest_free_page_hint(void *addr, size_t len);
/* migration/block.c */ diff --git a/migration/ram.c b/migration/ram.c
index 5e33e5c..e172798 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -2189,6 +2189,27 @@ static int ram_init_all(RAMState **rsp)
      return 0;
  }
This could do with some comments

OK, I'll add some.


+void qemu_guest_free_page_hint(void *addr, size_t len)
+{
+    RAMBlock *block;
+    ram_addr_t offset;
+    size_t used_len, start, npages;
 From your use I think the addr and len are coming raw from the guest;
so we need to take some care.


Actually the "addr" here has been the host address that corresponds to the guest free page. It's from elem->in_sg[0].iov_base.


+        if (unlikely(offset + len > block->used_length)) {
I think to make that overflow safe, that should be:
   if (len > (block->used_length - offset)) {

But we'll need another test before it, because qemu_ram_block_from_host
seems to check max_length not used_length, so we need to check
for offset > block->used_length first

OK, how about adding an assert above, like this:

block = qemu_ram_block_from_host(addr, false, &offset);
assert (offset  < block->used_length );
if (!block)
    ...

The address corresponds to a guest free page, which means it should be within used_length. If not, something weird happens, I think we'd better to assert it in that case.

Best,
Wei



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