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Re: [Qemu-devel] [PATCH v2 6/8] migration/ram: ensure write persistence


From: Haozhong Zhang
Subject: Re: [Qemu-devel] [PATCH v2 6/8] migration/ram: ensure write persistence on loading normal pages to PMEM
Date: Wed, 7 Feb 2018 20:02:43 +0800
User-agent: NeoMutt/20171027

On 02/07/18 11:49 +0000, Dr. David Alan Gilbert wrote:
> * Haozhong Zhang (address@hidden) wrote:
> > When loading a normal page to persistent memory, load its data by
> > libpmem function pmem_memcpy_nodrain() instead of memcpy(). Combined
> > with a call to pmem_drain() at the end of memory loading, we can
> > guarantee all those normal pages are persistenly loaded to PMEM.
> > 
> > Signed-off-by: Haozhong Zhang <address@hidden>
> > ---
> >  include/migration/qemu-file-types.h |  1 +
> >  include/qemu/pmem.h                 |  6 ++++++
> >  migration/qemu-file.c               | 41 
> > ++++++++++++++++++++++++++++---------
> >  migration/ram.c                     |  6 +++++-
> >  4 files changed, 43 insertions(+), 11 deletions(-)
> > 
> > diff --git a/include/migration/qemu-file-types.h 
> > b/include/migration/qemu-file-types.h
> > index bd6d7dd7f9..bb5c547498 100644
> > --- a/include/migration/qemu-file-types.h
> > +++ b/include/migration/qemu-file-types.h
> > @@ -34,6 +34,7 @@ void qemu_put_be16(QEMUFile *f, unsigned int v);
> >  void qemu_put_be32(QEMUFile *f, unsigned int v);
> >  void qemu_put_be64(QEMUFile *f, uint64_t v);
> >  size_t qemu_get_buffer(QEMUFile *f, uint8_t *buf, size_t size);
> > +size_t qemu_get_buffer_to_pmem(QEMUFile *f, uint8_t *buf, size_t size);
> >  
> >  int qemu_get_byte(QEMUFile *f);
> >  
> > diff --git a/include/qemu/pmem.h b/include/qemu/pmem.h
> > index 861d8ecc21..77ee1fc4eb 100644
> > --- a/include/qemu/pmem.h
> > +++ b/include/qemu/pmem.h
> > @@ -26,6 +26,12 @@ pmem_memcpy_persist(void *pmemdest, const void *src, 
> > size_t len)
> >      return memcpy(pmemdest, src, len);
> >  }
> >  
> > +static inline void *
> > +pmem_memcpy_nodrain(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);
> > diff --git a/migration/qemu-file.c b/migration/qemu-file.c
> > index 2ab2bf362d..7e573010d9 100644
> > --- a/migration/qemu-file.c
> > +++ b/migration/qemu-file.c
> > @@ -26,6 +26,7 @@
> >  #include "qemu-common.h"
> >  #include "qemu/error-report.h"
> >  #include "qemu/iov.h"
> > +#include "qemu/pmem.h"
> >  #include "migration.h"
> >  #include "qemu-file.h"
> >  #include "trace.h"
> > @@ -471,15 +472,8 @@ size_t qemu_peek_buffer(QEMUFile *f, uint8_t **buf, 
> > size_t size, size_t offset)
> >      return size;
> >  }
> >  
> > -/*
> > - * Read 'size' bytes of data from the file into buf.
> > - * 'size' can be larger than the internal buffer.
> > - *
> > - * It will return size bytes unless there was an error, in which case it 
> > will
> > - * return as many as it managed to read (assuming blocking fd's which
> > - * all current QEMUFile are)
> > - */
> > -size_t qemu_get_buffer(QEMUFile *f, uint8_t *buf, size_t size)
> > +static size_t
> > +qemu_get_buffer_common(QEMUFile *f, uint8_t *buf, size_t size, bool 
> > is_pmem)
> >  {
> >      size_t pending = size;
> >      size_t done = 0;
> > @@ -492,7 +486,11 @@ size_t qemu_get_buffer(QEMUFile *f, uint8_t *buf, 
> > size_t size)
> >          if (res == 0) {
> >              return done;
> >          }
> > -        memcpy(buf, src, res);
> > +        if (!is_pmem) {
> > +            memcpy(buf, src, res);
> > +        } else {
> > +            pmem_memcpy_nodrain(buf, src, res);
> > +        }
> 
> I see why you're doing it, but again I'm surprised it's ended up having
> to modify qemu-file.

Well, the current programming model of pmem requires users to take
special care on the data persistence, so QEMU (as a user of pmem) has
to do that as well.

> 
> >          qemu_file_skip(f, res);
> >          buf += res;
> >          pending -= res;
> > @@ -501,6 +499,29 @@ size_t qemu_get_buffer(QEMUFile *f, uint8_t *buf, 
> > size_t size)
> >      return done;
> >  }
> >  
> > +/*
> > + * Read 'size' bytes of data from the file into buf.
> > + * 'size' can be larger than the internal buffer.
> > + *
> > + * It will return size bytes unless there was an error, in which case it 
> > will
> > + * return as many as it managed to read (assuming blocking fd's which
> > + * all current QEMUFile are)
> > + */
> > +size_t qemu_get_buffer(QEMUFile *f, uint8_t *buf, size_t size)
> > +{
> > +    return qemu_get_buffer_common(f, buf, size, false);
> > +}
> > +
> > +/*
> > + * Mostly the same as qemu_get_buffer(), except that
> > + * 1) it's for the case that 'buf' is in the persistent memory, and
> > + * 2) it takes necessary operations to ensure the data persistence in 
> > 'buf'.
> > + */
> > +size_t qemu_get_buffer_to_pmem(QEMUFile *f, uint8_t *buf, size_t size)
> > +{
> > +    return qemu_get_buffer_common(f, buf, size, true);
> > +}
> > +
> >  /*
> >   * Read 'size' bytes of data from the file.
> >   * 'size' can be larger than the internal buffer.
> > diff --git a/migration/ram.c b/migration/ram.c
> > index 5a0e503818..5a79bbff64 100644
> > --- a/migration/ram.c
> > +++ b/migration/ram.c
> > @@ -2950,7 +2950,11 @@ static int ram_load(QEMUFile *f, void *opaque, int 
> > version_id)
> >              break;
> >  
> >          case RAM_SAVE_FLAG_PAGE:
> > -            qemu_get_buffer(f, host, TARGET_PAGE_SIZE);
> > +            if (!is_pmem) {
> > +                qemu_get_buffer(f, host, TARGET_PAGE_SIZE);
> > +            } else {
> > +                qemu_get_buffer_to_pmem(f, host, TARGET_PAGE_SIZE);
> > +            }
> 
> can you just expose qemu_get_buffer_common instead of making it static
> and just pass in the is_pmem flag.

Or perhaps passing a memcpyfunc function similarly to your comment on
patch 5.

Thanks,
Haozhong

> 
> Dave
> 
> >              break;
> >  
> >          case RAM_SAVE_FLAG_COMPRESS_PAGE:
> > -- 
> > 2.14.1
> > 
> --
> Dr. David Alan Gilbert / address@hidden / Manchester, UK



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