migration: Introduce postcopy channels on dest node

Postcopy handles huge pages in a special way that currently we can only have
one "channel" to transfer the page.

It's because when we install pages using UFFDIO_COPY, we need to have the whole
huge page ready, it also means we need to have a temp huge page when trying to
receive the whole content of the page.

Currently all maintainance around this tmp page is global: firstly we'll
allocate a temp huge page, then we maintain its status mostly within
ram_load_postcopy().

To enable multiple channels for postcopy, the first thing we need to do is to
prepare N temp huge pages as caching, one for each channel.

Meanwhile we need to maintain the tmp huge page status per-channel too.

To give some example, some local variables maintained in ram_load_postcopy()
are listed; they are responsible for maintaining temp huge page status:

  - all_zero:     this keeps whether this huge page contains all zeros
  - target_pages: this counts how many target pages have been copied
  - host_page:    this keeps the host ptr for the page to install

Move all these fields to be together with the temp huge pages to form a new
structure called PostcopyTmpPage.  Then for each (future) postcopy channel, we
need one structure to keep the state around.

For vanilla postcopy, obviously there's only one channel.  It contains both
precopy and postcopy pages.

This patch teaches the dest migration node to start realize the possible number
of postcopy channels by introducing the "postcopy_channels" variable.  Its
value is calculated when setup postcopy on dest node (during POSTCOPY_LISTEN
phase).

Vanilla postcopy will have channels=1, but when postcopy-preempt capability is
enabled (in the future), we will boost it to 2 because even during partial
sending of a precopy huge page we still want to preempt it and start sending
the postcopy requested page right away (so we start to keep two temp huge
pages; more if we want to enable multifd).  In this patch there's a TODO marked
for that; so far the channels is always set to 1.

We need to send one "host huge page" on one channel only and we cannot split
them, because otherwise the data upon the same huge page can locate on more
than one channel so we need more complicated logic to manage.  One temp host
huge page for each channel will be enough for us for now.

Postcopy will still always use the index=0 huge page even after this patch.
However it prepares for the latter patches where it can start to use multiple
channels (which needs src intervention, because only src knows which channel we
should use).

Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Peter Xu <peterx@redhat.com>
Message-Id: <20220301083925.33483-5-peterx@redhat.com>
Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
  dgilbert: Fixed up long line

1 files changed, 35 insertions, 1 deletions

diff --git a/migration/migration.h b/migration/migration.h
index 8130b703eb..42c7395094 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -45,6 +45,24 @@ struct PostcopyBlocktimeContext;
  */
 #define CLEAR_BITMAP_SHIFT_MAX            31
 
+/* This is an abstraction of a "temp huge page" for postcopy's purpose */
+typedef struct {
+    /*
+     * This points to a temporary huge page as a buffer for UFFDIO_COPY.  It's
+     * mmap()ed and needs to be freed when cleanup.
+     */
+    void *tmp_huge_page;
+    /*
+     * This points to the host page we're going to install for this temp page.
+     * It tells us after we've received the whole page, where we should put it.
+     */
+    void *host_addr;
+    /* Number of small pages copied (in size of TARGET_PAGE_SIZE) */
+    unsigned int target_pages;
+    /* Whether this page contains all zeros */
+    bool all_zero;
+} PostcopyTmpPage;
+
 /* State for the incoming migration */
 struct MigrationIncomingState {
     QEMUFile *from_src_file;
@@ -81,7 +99,22 @@ struct MigrationIncomingState {
     QemuMutex rp_mutex;    /* We send replies from multiple threads */
     /* RAMBlock of last request sent to source */
     RAMBlock *last_rb;
-    void     *postcopy_tmp_page;
+    /*
+     * Number of postcopy channels including the default precopy channel, so
+     * vanilla postcopy will only contain one channel which contain both
+     * precopy and postcopy streams.
+     *
+     * This is calculated when the src requests to enable postcopy but before
+     * it starts.  Its value can depend on e.g. whether postcopy preemption is
+     * enabled.
+     */
+    unsigned int postcopy_channels;
+    /*
+     * An array of temp host huge pages to be used, one for each postcopy
+     * channel.
+     */
+    PostcopyTmpPage *postcopy_tmp_pages;
+    /* This is shared for all postcopy channels */
     void     *postcopy_tmp_zero_page;
     /* PostCopyFD's for external userfaultfds & handlers of shared memory */
     GArray   *postcopy_remote_fds;
@@ -391,5 +424,6 @@ bool migration_rate_limit(void);
 void migration_cancel(const Error *error);
 
 void populate_vfio_info(MigrationInfo *info);
+void postcopy_temp_page_reset(PostcopyTmpPage *tmp_page);
 
 #endif