Re: [Qemu-devel] [PATCH v2 5/8] migration/ram: ensure write persistence on loading zero pages to PMEM

[Haozhong Zhang <haozhong.zhang@intel.com>]

On 02/07/18 11:38 +0000, Dr. David Alan Gilbert wrote:
> * Haozhong Zhang (haozhong.zhang@intel.com) wrote:
> > When loading a zero page, check whether it will be loaded to
> > persistent memory If yes, load it by libpmem function
> > pmem_memset_nodrain().  Combined with a call to pmem_drain() at the
> > end of RAM loading, we can guarantee all those zero pages are
> > persistently loaded.
> 
> I'm surprised pmem is this invasive to be honest;   I hadn't expected
> the need for special memcpy's etc everywhere.  We're bound to miss some.
> I assume there's separate kernel work needed to make postcopy work;
> certainly the patches here don't seem to touch the postcopy path.

This link at
https://wiki.qemu.org/Features/PostCopyLiveMigration#Conflicts shows
that postcopy with memory-backend-file requires kernel support. Can
you point me the details of the required kernel support, so that I can
understand what would be needed to NVDIMM postcopy?

> 
> > Depending on the host HW/SW configurations, pmem_drain() can be
> > "sfence".  Therefore, we do not call pmem_drain() after each
> > pmem_memset_nodrain(), or use pmem_memset_persist() (equally
> > pmem_memset_nodrain() + pmem_drain()), in order to avoid unnecessary
> > overhead.
> > 
> > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
> > ---
> >  include/qemu/pmem.h |  9 +++++++++
> >  migration/ram.c     | 34 +++++++++++++++++++++++++++++-----
> >  2 files changed, 38 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/qemu/pmem.h b/include/qemu/pmem.h
> > index 9017596ff0..861d8ecc21 100644
> > --- a/include/qemu/pmem.h
> > +++ b/include/qemu/pmem.h
> > @@ -26,6 +26,15 @@ pmem_memcpy_persist(void *pmemdest, const void *src, size_t len)
> >      return memcpy(pmemdest, src, len);
> >  }
> >  
> > +static inline void *pmem_memset_nodrain(void *pmemdest, int c, size_t len)
> > +{
> > +    return memset(pmemdest, c, len);
> > +}
> > +
> > +static inline void pmem_drain(void)
> > +{
> > +}
> > +
> >  #endif /* CONFIG_LIBPMEM */
> >  
> >  #endif /* !QEMU_PMEM_H */
> > diff --git a/migration/ram.c b/migration/ram.c
> > index cb1950f3eb..5a0e503818 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -49,6 +49,7 @@
> >  #include "qemu/rcu_queue.h"
> >  #include "migration/colo.h"
> >  #include "migration/block.h"
> > +#include "qemu/pmem.h"
> >  
> >  /***********************************************************/
> >  /* ram save/restore */
> > @@ -2467,6 +2468,20 @@ static inline void *host_from_ram_block_offset(RAMBlock *block,
> >      return block->host + offset;
> >  }
> >  
> > +static void ram_handle_compressed_common(void *host, uint8_t ch, uint64_t size,
> > +                                         bool is_pmem)
> 
> I don't think there's any advantage of splitting out this _common
> routine; lets just add the parameter to ram_handle_compressed.
> 
> > +{
> > +    if (!ch && is_zero_range(host, size)) {
> > +        return;
> > +    }
> > +
> > +    if (!is_pmem) {
> > +        memset(host, ch, size);
> > +    } else {
> > +        pmem_memset_nodrain(host, ch, size);
> > +    }
> 
> I'm wondering if it would be easier to pass in a memsetfunc ptr and call
> that (defualting to memset if it's NULL).

Yes, it would be more extensible if we have other special memory
devices in the future.

Thank,
Haozhong

> 
> > +}
> > +
> >  /**
> >   * ram_handle_compressed: handle the zero page case
> >   *
> > @@ -2479,9 +2494,7 @@ static inline void *host_from_ram_block_offset(RAMBlock *block,
> >   */
> >  void ram_handle_compressed(void *host, uint8_t ch, uint64_t size)
> >  {
> > -    if (ch != 0 || !is_zero_range(host, size)) {
> > -        memset(host, ch, size);
> > -    }
> > +    return ram_handle_compressed_common(host, ch, size, false);
> >  }
> >  
> >  static void *do_data_decompress(void *opaque)
> > @@ -2823,6 +2836,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
> >      bool postcopy_running = postcopy_is_running();
> >      /* ADVISE is earlier, it shows the source has the postcopy capability on */
> >      bool postcopy_advised = postcopy_is_advised();
> > +    bool need_pmem_drain = false;
> >  
> >      seq_iter++;
> >  
> > @@ -2848,6 +2862,8 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
> >          ram_addr_t addr, total_ram_bytes;
> >          void *host = NULL;
> >          uint8_t ch;
> > +        RAMBlock *block = NULL;
> > +        bool is_pmem = false;
> >  
> >          addr = qemu_get_be64(f);
> >          flags = addr & ~TARGET_PAGE_MASK;
> > @@ -2864,7 +2880,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
> >  
> >          if (flags & (RAM_SAVE_FLAG_ZERO | RAM_SAVE_FLAG_PAGE |
> >                       RAM_SAVE_FLAG_COMPRESS_PAGE | RAM_SAVE_FLAG_XBZRLE)) {
> > -            RAMBlock *block = ram_block_from_stream(f, flags);
> > +            block = ram_block_from_stream(f, flags);
> >  
> >              host = host_from_ram_block_offset(block, addr);
> >              if (!host) {
> > @@ -2874,6 +2890,9 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
> >              }
> >              ramblock_recv_bitmap_set(block, host);
> >              trace_ram_load_loop(block->idstr, (uint64_t)addr, flags, host);
> > +
> > +            is_pmem = ramblock_is_pmem(block);
> > +            need_pmem_drain = need_pmem_drain || is_pmem;
> >          }
> >  
> >          switch (flags & ~RAM_SAVE_FLAG_CONTINUE) {
> > @@ -2927,7 +2946,7 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
> >  
> >          case RAM_SAVE_FLAG_ZERO:
> >              ch = qemu_get_byte(f);
> > -            ram_handle_compressed(host, ch, TARGET_PAGE_SIZE);
> > +            ram_handle_compressed_common(host, ch, TARGET_PAGE_SIZE, is_pmem);
> >              break;
> >  
> >          case RAM_SAVE_FLAG_PAGE:
> > @@ -2970,6 +2989,11 @@ static int ram_load(QEMUFile *f, void *opaque, int version_id)
> >      }
> >  
> >      wait_for_decompress_done();
> > +
> > +    if (need_pmem_drain) {
> > +        pmem_drain();
> > +    }
> > +
> >      rcu_read_unlock();
> >      trace_ram_load_complete(ret, seq_iter);
> >      return ret;
> > -- 
> > 2.14.1
> 
> Dave
> 
> > 
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK