Re: [PATCH v2 16/29] migration/ram: Add support for 'fixed-ram' migration restore

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Oct 23, 2023 at 05:35:55PM -0300, Fabiano Rosas wrote:
> From: Nikolay Borisov <nborisov@suse.com>
> 
> Add the necessary code to parse the format changes for the 'fixed-ram'
> capability.
> 
> One of the more notable changes in behavior is that in the 'fixed-ram'
> case ram pages are restored in one go rather than constantly looping
> through the migration stream.
> 
> Signed-off-by: Nikolay Borisov <nborisov@suse.com>
> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> ---
> (farosas) reused more of the common code by making the fixed-ram
> function take only one ramblock and calling it from inside
> parse_ramblock.
> ---
>  migration/ram.c | 93 +++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 93 insertions(+)
> 
> diff --git a/migration/ram.c b/migration/ram.c
> index 152a03604f..cea6971ab2 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -3032,6 +3032,32 @@ static void fixed_ram_insert_header(QEMUFile *file, RAMBlock *block)
>      qemu_put_buffer(file, (uint8_t *) header, header_size);
>  }
>  
> +static int fixed_ram_read_header(QEMUFile *file, struct FixedRamHeader *header)
> +{
> +    size_t ret, header_size = sizeof(struct FixedRamHeader);
> +
> +    ret = qemu_get_buffer(file, (uint8_t *)header, header_size);
> +    if (ret != header_size) {
> +        return -1;
> +    }
> +
> +    /* migration stream is big-endian */
> +    be32_to_cpus(&header->version);
> +
> +    if (header->version > FIXED_RAM_HDR_VERSION) {
> +        error_report("Migration fixed-ram capability version mismatch (expected %d, got %d)",
> +                     FIXED_RAM_HDR_VERSION, header->version);
> +        return -1;
> +    }
> +
> +    be64_to_cpus(&header->page_size);
> +    be64_to_cpus(&header->bitmap_offset);
> +    be64_to_cpus(&header->pages_offset);
> +
> +
> +    return 0;
> +}
> +
>  /*
>   * Each of ram_save_setup, ram_save_iterate and ram_save_complete has
>   * long-running RCU critical section.  When rcu-reclaims in the code
> @@ -3932,6 +3958,68 @@ void colo_flush_ram_cache(void)
>      trace_colo_flush_ram_cache_end();
>  }
>  
> +static void read_ramblock_fixed_ram(QEMUFile *f, RAMBlock *block,
> +                                    long num_pages, unsigned long *bitmap)
> +{
> +    unsigned long set_bit_idx, clear_bit_idx;
> +    unsigned long len;
> +    ram_addr_t offset;
> +    void *host;
> +    size_t read, completed, read_len;
> +
> +    for (set_bit_idx = find_first_bit(bitmap, num_pages);
> +         set_bit_idx < num_pages;
> +         set_bit_idx = find_next_bit(bitmap, num_pages, clear_bit_idx + 1)) {
> +
> +        clear_bit_idx = find_next_zero_bit(bitmap, num_pages, set_bit_idx + 1);
> +
> +        len = TARGET_PAGE_SIZE * (clear_bit_idx - set_bit_idx);
> +        offset = set_bit_idx << TARGET_PAGE_BITS;
> +
> +        for (read = 0, completed = 0; completed < len; offset += read) {
> +            host = host_from_ram_block_offset(block, offset);
> +            read_len = MIN(len, TARGET_PAGE_SIZE);
> +
> +            read = qemu_get_buffer_at(f, host, read_len,
> +                                      block->pages_offset + offset);
> +            completed += read;
> +        }
> +    }
> +}
> +
> +static int parse_ramblock_fixed_ram(QEMUFile *f, RAMBlock *block, ram_addr_t length)
> +{
> +    g_autofree unsigned long *bitmap = NULL;
> +    struct FixedRamHeader header;
> +    size_t bitmap_size;
> +    long num_pages;
> +    int ret = 0;
> +
> +    ret = fixed_ram_read_header(f, &header);
> +    if (ret < 0) {
> +        error_report("Error reading fixed-ram header");
> +        return -EINVAL;
> +    }
> +
> +    block->pages_offset = header.pages_offset;

Do you think it is worth sanity checking that 'pages_offset' is aligned
in some way.

It is nice that we have flexibility to change the alignment in future
if we find 1 MB is not optimal, so I wouldn't want to force 1MB align
check htere. Perhaps we could at least sanity check for alignment at
TARGET_PAGE_SIZE, to detect a gross data corruption problem ?

> +    num_pages = length / header.page_size;
> +    bitmap_size = BITS_TO_LONGS(num_pages) * sizeof(unsigned long);
> +
> +    bitmap = g_malloc0(bitmap_size);
> +    if (qemu_get_buffer_at(f, (uint8_t *)bitmap, bitmap_size,
> +                           header.bitmap_offset) != bitmap_size) {
> +        error_report("Error parsing dirty bitmap");

s/parsing/reading/ since we're not actually parsing any semantic
info here.

> +        return -EINVAL;
> +    }
> +
> +    read_ramblock_fixed_ram(f, block, num_pages, bitmap);
> +
> +    /* Skip pages array */
> +    qemu_set_offset(f, block->pages_offset + length, SEEK_SET);
> +
> +    return ret;
> +}
> +
>  static int parse_ramblock(QEMUFile *f, RAMBlock *block, ram_addr_t length)
>  {
>      int ret = 0;
> @@ -3940,6 +4028,10 @@ static int parse_ramblock(QEMUFile *f, RAMBlock *block, ram_addr_t length)
>  
>      assert(block);
>  
> +    if (migrate_fixed_ram()) {
> +        return parse_ramblock_fixed_ram(f, block, length);
> +    }
> +
>      if (!qemu_ram_is_migratable(block)) {
>          error_report("block %s should not be migrated !", block->idstr);
>          return -EINVAL;
> @@ -4142,6 +4234,7 @@ static int ram_load_precopy(QEMUFile *f)
>                  migrate_multifd_flush_after_each_section()) {
>                  multifd_recv_sync_main();
>              }
> +
>              break;

Spurious whitespace


>          case RAM_SAVE_FLAG_HOOK:
>              ret = rdma_registration_handle(f);
> -- 
> 2.35.3
> 

With regards,
Daniel
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