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Re: [Qemu-devel] [PATCH v7 34/42] Postcopy: Use helpers to map pages dur


From: Amit Shah
Subject: Re: [Qemu-devel] [PATCH v7 34/42] Postcopy: Use helpers to map pages during migration
Date: Mon, 27 Jul 2015 13:09:34 +0530

On (Tue) 16 Jun 2015 [11:26:47], Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> In postcopy, the destination guest is running at the same time
> as it's receiving pages; as we receive new pages we must put
> them into the guests address space atomically to avoid a running
> CPU accessing a partially written page.
> 
> Use the helpers in postcopy-ram.c to map these pages.
> 
> qemu_get_buffer_less_copy is used to avoid a copy out of qemu_file
> in the case that postcopy is going to do a copy anyway.
> 
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>


> @@ -1881,6 +1890,16 @@ static int ram_load(QEMUFile *f, void *opaque, int 
> version_id)
>      int flags = 0, ret = 0;
>      static uint64_t seq_iter;
>      int len = 0;
> +    /*
> +     * System is running in postcopy mode, page inserts to host memory must 
> be
> +     * atomic
> +     */

*If* system is running in postcopy mode ....

> +    MigrationIncomingState *mis = migration_incoming_get_current();
> +    bool postcopy_running = postcopy_state_get(mis) >=
> +                            POSTCOPY_INCOMING_LISTENING;
> +    void *postcopy_host_page = NULL;
> +    bool postcopy_place_needed = false;
> +    bool matching_page_sizes = qemu_host_page_size == TARGET_PAGE_SIZE;
>  
>      seq_iter++;
>  
> @@ -1896,13 +1915,57 @@ static int ram_load(QEMUFile *f, void *opaque, int 
> version_id)
>      rcu_read_lock();
>      while (!ret && !(flags & RAM_SAVE_FLAG_EOS)) {
>          ram_addr_t addr, total_ram_bytes;
> -        void *host;
> +        void *host = 0;
> +        void *page_buffer = 0;
> +        void *postcopy_place_source = 0;
>          uint8_t ch;
> +        bool all_zero = false;
>  
>          addr = qemu_get_be64(f);
>          flags = addr & ~TARGET_PAGE_MASK;
>          addr &= TARGET_PAGE_MASK;
>  
> +        if (flags & (RAM_SAVE_FLAG_COMPRESS | RAM_SAVE_FLAG_PAGE |
> +                     RAM_SAVE_FLAG_XBZRLE)) {
> +            host = host_from_stream_offset(f, mis, addr, flags);
> +            if (!host) {
> +                error_report("Illegal RAM offset " RAM_ADDR_FMT, addr);
> +                ret = -EINVAL;
> +                break;
> +            }

So the host_from_stream_offset was moved here from below.  One
invocation below is still left, which is a bug..

> +            if (!postcopy_running) {
> +                page_buffer = host;
> +            } else {

Instead of this, can we just do:

           page_buffer = host;
           if (postcopy_running) {

> +                /*
> +                 * Postcopy requires that we place whole host pages 
> atomically.
> +                 * To make it atomic, the data is read into a temporary page
> +                 * that's moved into place later.
> +                 * The migration protocol uses,  possibly smaller, 
> target-pages
> +                 * however the source ensures it always sends all the 
> components
> +                 * of a host page in order.
> +                 */
> +                if (!postcopy_host_page) {
> +                    postcopy_host_page = postcopy_get_tmp_page(mis);
> +                }
> +                page_buffer = postcopy_host_page +
> +                              ((uintptr_t)host & ~qemu_host_page_mask);
> +                /* If all TP are zero then we can optimise the place */
> +                if (!((uintptr_t)host & ~qemu_host_page_mask)) {
> +                    all_zero = true;
> +                }
> +
> +                /*
> +                 * If it's the last part of a host page then we place the 
> host
> +                 * page
> +                 */
> +                postcopy_place_needed = (((uintptr_t)host + 
> TARGET_PAGE_SIZE) &
> +                                         ~qemu_host_page_mask) == 0;
> +                postcopy_place_source = postcopy_host_page;
> +            }
> +        } else {
> +            postcopy_place_needed = false;
> +        }

... and similar for postcopy_place_needed as well?  It becomes much
easier to read.

>          case RAM_SAVE_FLAG_COMPRESS_PAGE:
> -            host = host_from_stream_offset(f, addr, flags);
> +            all_zero = false;
> +            if (postcopy_running) {
> +                error_report("Compressed RAM in postcopy mode @%zx\n", addr);
> +                return -EINVAL;
> +            }
> +            host = host_from_stream_offset(f, mis, addr, flags);

This line should go (as mentioned above)?


                Amit



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