On Mon, Jan 26, 2026 at 06:37:31PM -0300, Fabiano Rosas wrote:
> Lukas Straub <[email protected]> writes:
>
> >> > +void multifd_colo_prepare_recv(MultiFDRecvParams *p)
> >> > +{
> >> > + /*
> >> > + * While we're still in precopy state (not yet in colo state), we
> >> > copy
> >> > + * received pages to both guest and cache. No need to set dirty
> >> > bits,
> >> > + * since guest and cache memory are in sync.
> >> > + */
> >> > + if (migration_incoming_in_colo_state()) {
> >>
> >> What's the relationship between migration_incoming_in_colo_state() and
> >> migration_incoming_colo_enabled()? ram_load_precopy() checks both. Would
> >> migration_incoming_colo_enabled affect multifd as well?
> >
> > So first off migration_incoming_colo_enabled() and migrate_colo()
> > are equivalent in practice since
> > 121ccedc2b migration: block incoming colo when capability is disabled
> >
> > (I have some cleanup patches lying around, but that will be for later)
> >
>
> Ok, I think those are important because when having multifd and
> non-multifd code for the same feature, it's useful to be able to compare
> the two. So some degree of uniformity would be nice.
I second. We can drop those in this series before adding multifd support,
likely together with MIG_CMD_ENABLE_COLO as well; I don't think COLO needs
to worry about old binaries. It should always use the same QEMU binary on
both sides.
The patch needs to rename MIG_CMD_ENABLE_COLO to MIG_CMD_DEPRECATED_0 or
something, to make the rest MIG_CMD compatible to old binaries, though.
>
> > For colo, we do normal precopy migration and at the end we go into colo
> > state and then
> > migration_incoming_in_colo_state() will be true.
> >
> > Here we check migrate_colo() outside of these functions as Peter
> > requested that in a previous version.
> >
> >>
> >> The multifd recv threads will be running until after
> >> process_incoming_migration_bh(), which is when
> >> migration_incoming_disable_colo() runs.
> >
> > That is not an issue as we use migrate_colo() here.
> >
> >>
> >> Also, is the colo_cache guaranteed to be around until multifd threads
> >> exit?
> >
> > This is an issue. I will fix it in the next version.
> >
> >>
> >> > + colo_record_bitmap(p->block, p->normal, p->normal_num);
> >> > + colo_record_bitmap(p->block, p->zero, p->zero_num);
> >> > + }
> >> > +}
> >> > +
> >> > +void multifd_colo_process_recv(MultiFDRecvParams *p)
> >> > +{
> >> > + if (!migration_incoming_in_colo_state()) {
> >> > + for (int i = 0; i < p->normal_num; i++) {
> >> > + void *guest = p->block->host + p->normal[i];
> >> > + void *cache = p->host + p->normal[i];
> >> > + memcpy(guest, cache, multifd_ram_page_size());
> >> > + }
> >>
> >> I see some differences between what ram.c does and what multifd will do
> >> after this patch regarding which flags are checked and order of copies
> >> (code below):
> >>
> >> ram.c:
> >>
> >> - migration_incoming_colo_enabled && migration_incoming_in_colo_state:
> >> Reads from stream into colo_cache.
> >>
> >> - migration_incoming_colo_enabled && !migration_incoming_in_colo_state:
> >> Reads from stream into guest and then memcpy into colo_cache.
> >>
> >> - !migration_incoming_colo_enabled
> >> Reads from stream into guest.
> >>
> >> multifd.c:
> >>
> >> - migrate_colo:
> >> Reads from stream into colo_cache.
> >>
> >> - !migration_incoming_in_colo_state:
> >> memcpy from colo_cache into guest.
> >>
> >> - !migration_incoming_colo_enabled
> >> ???
> >>
> >> The resulting state should be the same, but I wonder if we want to i) use
> >> the same checks in multifd
> >
> > migration_incoming_colo_enabled() shouldn't even exist anymore, so I'm
> > not using it here. migrate_colo() is much easier to reason about.
> >
> >> and ii) when not in colo state, copy first
> >> into guest (using readv) and later memcpy into the colo_cache.
> >
> > I think it is easier the way it is now.
> >
> >>
> >> ---
> >> ram.c:
> >>
> >> host = host_from_ram_block_offset(block, addr);
> >> if (migration_incoming_colo_enabled()) {
> >> if (migration_incoming_in_colo_state()) {
> >> host = colo_cache_from_block_offset(block, addr, true);
> >> } else {
> >> host_bak = colo_cache_from_block_offset(block, addr, false);
> >> }
> >> }
> >> qemu_get_buffer(f, host, TARGET_PAGE_SIZE);
> >> if (host_bak) {
> >> memcpy(host_bak, host, TARGET_PAGE_SIZE);
> >> }
> >>
> >> multifd:
> >>
> >> if (migrate_colo()) {
> >> p->host = p->block->colo_cache;
> >> }
> >>
> >> for (int i = 0; i < p->normal_num; i++) {
> >> p->iov[i].iov_base = p->host + p->normal[i];
> >> }
> >> return qio_channel_readv_all(p->c, p->iov, p->normal_num, errp);
> >>
> >> if (!migration_incoming_in_colo_state()) {
> >> for (int i = 0; i < p->normal_num; i++) {
> >> void *guest = p->block->host + p->normal[i];
> >> void *cache = p->host + p->normal[i];
> >> memcpy(guest, cache, multifd_ram_page_size());
> >> }
> >> }
> >> ---
> >>
> >> > + for (int i = 0; i < p->zero_num; i++) {
> >> > + void *guest = p->block->host + p->zero[i];
> >> > + memset(guest, 0, multifd_ram_page_size());
> >> > + }
> >>
> >> At multifd_nocomp_recv, there will be a call to
> >> multifd_recv_zero_page_process(), which by that point will have p->host
> >> == p->block->colo_cache, so it looks like that function will do some
> >> zero page processing in the colo_cache, setting the rb->receivedmap for
> >> pages in the colo_cache and potentially also doing a memcpy. Is this
> >> intended?
> >
> > rb->receivedmap is only for postcopy, right? So it doesn't apply with
> > colo.
> >
>
> It's not anymore since commit 5ef7e26bdb ("migration/multifd: solve zero
> page causing multiple page faults"). So it seems we might be doing extra
> work on top of the colo_cache.
IIUC not extra, but exactly what will be needed.
The logic was about "in a vanilla precopy, if we see one page arriving the
1st time we don't need to zero the buffer because the buffer should be zero
allocated".
In COLO's case, COLO always puts RAM data into colo_cache, hence it should
apply to colo_cache too, avoiding unnecessary memset() for colo_cache
instead.
E.g. colo_cache is allocated from qemu_anon_ram_alloc(), it's also
guaranteed to be zeros when never touched.
>
> >>
> >> I'm thinking that maybe it would overall be better to hook colo directly
> >> in to multifd_nocomp_recv:
> >
> > But then it will only work for nocomp, right? It feels like the wrong
> > level of abstraction to me.
> >
>
> Ah, nocomp != ram indeed.
>
> >>
> >> static int multifd_nocomp_recv(MultiFDRecvParams *p, Error **errp)
> >> {
> >> uint32_t flags;
> >>
> >> if (migrate_mapped_ram()) {
> >> return multifd_file_recv_data(p, errp);
> >> }
> >>
> >> flags = p->flags & MULTIFD_FLAG_COMPRESSION_MASK;
> >>
> >> if (flags != MULTIFD_FLAG_NOCOMP) {
> >> error_setg(errp, "multifd %u: flags received %x flags expected %x",
> >> p->id, flags, MULTIFD_FLAG_NOCOMP);
> >> return -1;
> >> }
> >>
> >> + if (migration_incoming_colo_enabled() &&
> >> migration_incoming_in_colo_state()) {
> >> + p->host = p->block->colo_cache;
> >> + } // or else{}, depending on how deal with zero pages in the cache
> >>
> >> multifd_recv_zero_page_process(p);
> >>
> >> if (!p->normal_num) {
> >> return 0;
> >> }
> >>
> >> for (int i = 0; i < p->normal_num; i++) {
> >> p->iov[i].iov_base = p->host + p->normal[i];
> >> p->iov[i].iov_len = multifd_ram_page_size();
> >> ramblock_recv_bitmap_set_offset(p->block, p->normal[i]);
> >> }
> >> + ret = qio_channel_readv_all(p->c, p->iov, p->normal_num, errp);
> >> + if (ret != 0) {
> >> + return ret;
> >> + }
> >> +
> >> + if (migration_incoming_colo_enabled()) {
> >> + multifd_colo_process_recv();
> >> + }
> >>
> >> return ret;
> >> }
> >>
> >>
> >> > + }
> >> > +}
> >> > diff --git a/migration/multifd-colo.h b/migration/multifd-colo.h
> >> > new file mode 100644
> >> > index
> >> > 0000000000000000000000000000000000000000..82eaf3f48c47de2f090f9de52f9d57a337d4754a
> >> > --- /dev/null
> >> > +++ b/migration/multifd-colo.h
> >> > @@ -0,0 +1,26 @@
> >> > +/*
> >> > + * SPDX-License-Identifier: GPL-2.0-or-later
> >> > + *
> >> > + * multifd colo header
> >> > + *
> >> > + * Copyright (c) Lukas Straub <[email protected]>
> >> > + *
> >> > + * This work is licensed under the terms of the GNU GPL, version 2 or
> >> > later.
> >> > + * See the COPYING file in the top-level directory.
> >> > + */
> >> > +
> >> > +#ifndef QEMU_MIGRATION_MULTIFD_COLO_H
> >> > +#define QEMU_MIGRATION_MULTIFD_COLO_H
> >> > +
> >> > +#ifdef CONFIG_REPLICATION
> >> > +
> >> > +void multifd_colo_prepare_recv(MultiFDRecvParams *p);
> >> > +void multifd_colo_process_recv(MultiFDRecvParams *p);
> >> > +
> >> > +#else
> >> > +
> >> > +static inline void multifd_colo_prepare_recv(MultiFDRecvParams *p) {}
> >> > +static inline void multifd_colo_process_recv(MultiFDRecvParams *p) {}
> >> > +
> >> > +#endif
> >> > +#endif
> >> > diff --git a/migration/multifd-nocomp.c b/migration/multifd-nocomp.c
> >> > index
> >> > 9be79b3b8e00371ebff9e112766c225bec260bf7..9f7a792fa761b3bc30b971b35f464103a61787f0
> >> > 100644
> >> > --- a/migration/multifd-nocomp.c
> >> > +++ b/migration/multifd-nocomp.c
> >> > @@ -16,6 +16,7 @@
> >> > #include "file.h"
> >> > #include "migration-stats.h"
> >> > #include "multifd.h"
> >> > +#include "multifd-colo.h"
> >> > #include "options.h"
> >> > #include "migration.h"
> >> > #include "qapi/error.h"
> >> > @@ -269,7 +270,6 @@ int multifd_ram_unfill_packet(MultiFDRecvParams *p,
> >> > Error **errp)
> >> > return -1;
> >> > }
> >> >
> >> > - p->host = p->block->host;
> >> > for (i = 0; i < p->normal_num; i++) {
> >> > uint64_t offset = be64_to_cpu(packet->offset[i]);
> >> >
> >> > @@ -294,6 +294,14 @@ int multifd_ram_unfill_packet(MultiFDRecvParams *p,
> >> > Error **errp)
> >> > p->zero[i] = offset;
> >> > }
> >> >
> >> > + if (migrate_colo()) {
> >> > + multifd_colo_prepare_recv(p);
> >> > + assert(p->block->colo_cache);
> >> > + p->host = p->block->colo_cache;
> >>
> >> Can't you just use p->block->colo_cache later? I don't see why p->host
> >> needs to be set beforehand even in the non-colo case.
> >
> > We should not touch the guest ram directly while in colo state, since
> > the incoming guest is running and we either want to receive and apply a
> > whole checkpoint with all ram into colo cache and all device state,
> > or if anything goes wrong during checkpointing, keep the currently
> > running guest on the incoming side in pristine state.
> >
>
> I was asking about setting p->host at this specific point. I don't think
> any of this fits the unfill function. However, I see those were
> suggested by Peter so let's not go back and forth.
Actually I don't know why p->host existed before this work; IIUC we could
have always used p->block->host. Maybe when Juan was developing this Juan
kept COLO in mind; or maybe Juan wanted to avoid frequent p->block pointer
reference.
IIUC, we could remove p->host, but when we need to access "the buffer of
the ramblock" we'll need to call a helper to fetch that (either ramblock's
buffer, or colo_cache, per migrate_colo()). And it might be slightly
slower than p->host indeed.
>
> > I have written more about colo migration here:
> >
> > https://lore.kernel.org/qemu-devel/20260117204913.584e1829@penguin/
> > https://lore.kernel.org/qemu-devel/[email protected]/
> >
> >>
> >> > + } else {
> >> > + p->host = p->block->host;
> >> > + }
> >> > +
> >> > return 0;
> >> > }
> >> >
> >> > diff --git a/migration/multifd.c b/migration/multifd.c
> >> > index
> >> > 332e6fc58053462419f3171f6c320ac37648ef7b..220ed8564960fdabc58e4baa069dd252c8ad293c
> >> > 100644
> >> > --- a/migration/multifd.c
> >> > +++ b/migration/multifd.c
> >> > @@ -29,6 +29,7 @@
> >> > #include "qemu-file.h"
> >> > #include "trace.h"
> >> > #include "multifd.h"
> >> > +#include "multifd-colo.h"
> >> > #include "options.h"
> >> > #include "qemu/yank.h"
> >> > #include "io/channel-file.h"
> >> > @@ -1258,6 +1259,13 @@ static int
> >> > multifd_ram_state_recv(MultiFDRecvParams *p, Error **errp)
> >> > int ret;
> >> >
> >> > ret = multifd_recv_state->ops->recv(p, errp);
> >> > + if (ret != 0) {
> >> > + return ret;
> >> > + }
> >> > +
> >> > + if (migrate_colo()) {
> >> > + multifd_colo_process_recv(p);
> >> > + }
> >>
> >> Either put all of colo hooks in multifd.c or multifd-nocomp.c. I think
> >> the latter is more appropriate as we have mapped_ram already in
> >> there. Let's drop patch 3 and put this in multifd_nocomp_recv().
> >
> > Again, it also should work with compression and multifd_nocomp_recv()
> > is for nocomp only.
> >
> >>
> >> >
> >> > return ret;
> >> > }
> >> > diff --git a/migration/multifd.h b/migration/multifd.h
> >> > index
> >> > 89a395aef2b09a6762c45b5361e0ab63256feff6..fbc35702b062fdc3213ce92baed35994f5967c2b
> >> > 100644
> >> > --- a/migration/multifd.h
> >> > +++ b/migration/multifd.h
> >> > @@ -279,7 +279,10 @@ typedef struct {
> >> > uint64_t packets_recved;
> >> > /* ramblock */
> >> > RAMBlock *block;
> >> > - /* ramblock host address */
> >> > + /*
> >> > + * Normally, it points to ramblock's host address. When COLO
> >> > + * is enabled, it points to the mirror cache for the ramblock.
> >> > + */
> >> > uint8_t *host;
> >> > /* buffers to recv */
> >> > struct iovec *iov;
>
--
Peter Xu
