Re: [Qemu-devel] [PATCH v2 7/8] migration/ram: ensure write persistence on loading compressed pages to PMEM

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Haozhong Zhang (haozhong.zhang@intel.com) wrote:
> When loading a compressed page to persistent memory, flush CPU cache
> after the data is decompressed. Combined with a call to pmem_drain()
> at the end of memory loading, we can guarantee those compressed pages
> are persistently loaded to PMEM.

Can you explain why this can use the flush and doesn't need the special
memset?

Dave

> Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> ---
>  include/qemu/pmem.h |  4 ++++
>  migration/ram.c     | 16 +++++++++++-----
>  2 files changed, 15 insertions(+), 5 deletions(-)
> 
> diff --git a/include/qemu/pmem.h b/include/qemu/pmem.h
> index 77ee1fc4eb..20e3f6e71d 100644
> --- a/include/qemu/pmem.h
> +++ b/include/qemu/pmem.h
> @@ -37,6 +37,10 @@ static inline void *pmem_memset_nodrain(void *pmemdest, int c, size_t len)
>      return memset(pmemdest, c, len);
>  }
>  
> +static inline void pmem_flush(const void *addr, size_t len)
> +{
> +}
> +
>  static inline void pmem_drain(void)
>  {
>  }
> diff --git a/migration/ram.c b/migration/ram.c
> index 5a79bbff64..924d2b9537 100644
> --- a/migration/ram.c
> +++ b/migration/ram.c
> @@ -274,6 +274,7 @@ struct DecompressParam {
>      void *des;
>      uint8_t *compbuf;
>      int len;
> +    bool is_pmem;
>  };
>  typedef struct DecompressParam DecompressParam;
>  
> @@ -2502,7 +2503,7 @@ static void *do_data_decompress(void *opaque)
>      DecompressParam *param = opaque;
>      unsigned long pagesize;
>      uint8_t *des;
> -    int len;
> +    int len, rc;
>  
>      qemu_mutex_lock(&param->mutex);
>      while (!param->quit) {
> @@ -2518,8 +2519,11 @@ static void *do_data_decompress(void *opaque)
>               * not a problem because the dirty page will be retransferred
>               * and uncompress() won't break the data in other pages.
>               */
> -            uncompress((Bytef *)des, &pagesize,
> -                       (const Bytef *)param->compbuf, len);
> +            rc = uncompress((Bytef *)des, &pagesize,
> +                            (const Bytef *)param->compbuf, len);
> +            if (rc == Z_OK && param->is_pmem) {
> +                pmem_flush(des, len);
> +            }
>  
>              qemu_mutex_lock(&decomp_done_lock);
>              param->done = true;
> @@ -2605,7 +2609,8 @@ static void compress_threads_load_cleanup(void)
>  }
>  
>  static void decompress_data_with_multi_threads(QEMUFile *f,
> -                                               void *host, int len)
> +                                               void *host, int len,
> +                                               bool is_pmem)
>  {
>      int idx, thread_count;
>  
> @@ -2619,6 +2624,7 @@ static void decompress_data_with_multi_threads(QEMUFile *f,
>                  qemu_get_buffer(f, decomp_param[idx].compbuf, len);
>                  decomp_param[idx].des = host;
>                  decomp_param[idx].len = len;
> +                decomp_param[idx].is_pmem = is_pmem;
>                  qemu_cond_signal(&decomp_param[idx].cond);
>                  qemu_mutex_unlock(&decomp_param[idx].mutex);
>                  break;
> @@ -2964,7 +2970,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
>                  ret = -EINVAL;
>                  break;
>              }
> -            decompress_data_with_multi_threads(f, host, len);
> +            decompress_data_with_multi_threads(f, host, len, is_pmem);
>              break;
>  
>          case RAM_SAVE_FLAG_XBZRLE:
> -- 
> 2.14.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK