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migration: mapped-ram: handle zero pages

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Make mapped-ram compatible with loadvm snapshot restoring by explicitly
zeroing memory pages in this case.
Skip zeroing for -incoming and -loadvm migrations to preserve performance.

Signed-off-by: Marco Cavenati <Marco.Cavenati@eurecom.fr>
Link: https://lore.kernel.org/r/20251010115954.1995298-3-Marco.Cavenati@eurecom.fr
Signed-off-by: Peter Xu <peterx@redhat.com>
This commit is contained in:
Marco Cavenati 2025-10-10 13:59:54 +02:00 • committed by Peter Xu
parent 04a191cb36
commit 0ecd285824
2 changed files with 58 additions and 2 deletions
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1
migration/options.c
View file

@ -449,7 +449,6 @@ INITIALIZE_MIGRATE_CAPS_SET(check_caps_background_snapshot,
static const
INITIALIZE_MIGRATE_CAPS_SET(check_caps_savevm,
MIGRATION_CAPABILITY_MULTIFD,
MIGRATION_CAPABILITY_MAPPED_RAM,
);
static bool migrate_incoming_started(void)

59
migration/ram.c
View file

@ -4038,12 +4038,58 @@ static size_t ram_load_multifd_pages(void *host_addr, size_t size,
return size;
}
/**
* handle_zero_mapped_ram: Zero out a range of RAM pages if required during
* mapped-ram load
*
* Zeroing is only performed when restoring from a snapshot (HMP loadvm).
* During incoming migration or -loadvm cli snapshot load, the function is a
* no-op and returns true as in those cases the pages are already guaranteed to
* be zeroed.
*
* Returns: true on success, false on error (with @errp set).
* @from_bit_idx: Starting index relative to the map of the page (inclusive)
* @to_bit_idx: Ending index relative to the map of the page (exclusive)
*/
static bool handle_zero_mapped_ram(RAMBlock *block, unsigned long from_bit_idx,
unsigned long to_bit_idx, Error **errp)
{
ERRP_GUARD();
ram_addr_t offset;
size_t size;
void *host;
/*
* Zeroing is not needed for either -loadvm (RUN_STATE_PRELAUNCH), or
* -incoming (RUN_STATE_INMIGRATE).
*/
if (!runstate_check(RUN_STATE_RESTORE_VM)) {
return true;
}
if (from_bit_idx >= to_bit_idx) {
return true;
}
size = TARGET_PAGE_SIZE * (to_bit_idx - from_bit_idx);
offset = from_bit_idx << TARGET_PAGE_BITS;
host = host_from_ram_block_offset(block, offset);
if (!host) {
error_setg(errp, "zero page outside of ramblock %s range",
block->idstr);
return false;
}
ram_handle_zero(host, size);
return true;
}
static bool read_ramblock_mapped_ram(QEMUFile *f, RAMBlock *block,
long num_pages, unsigned long *bitmap,
Error **errp)
{
ERRP_GUARD();
unsigned long set_bit_idx, clear_bit_idx;
unsigned long set_bit_idx, clear_bit_idx = 0;
ram_addr_t offset;
void *host;
size_t read, unread, size;
@ -4052,6 +4098,12 @@ static bool read_ramblock_mapped_ram(QEMUFile *f, RAMBlock *block,
set_bit_idx < num_pages;
set_bit_idx = find_next_bit(bitmap, num_pages, clear_bit_idx + 1)) {
/* Zero pages */
if (!handle_zero_mapped_ram(block, clear_bit_idx, set_bit_idx, errp)) {
return false;
}
/* Non-zero pages */
clear_bit_idx = find_next_zero_bit(bitmap, num_pages, set_bit_idx + 1);
unread = TARGET_PAGE_SIZE * (clear_bit_idx - set_bit_idx);
@ -4083,6 +4135,11 @@ static bool read_ramblock_mapped_ram(QEMUFile *f, RAMBlock *block,
}
}
/* Handle trailing 0 pages */
if (!handle_zero_mapped_ram(block, clear_bit_idx, num_pages, errp)) {
return false;
}
return true;
err: