Re: [Qemu-devel] [PATCH 2/4] Memory: Enable writeback for given memory region

[Alex Bennée]

Beata Michalska <address@hidden> writes:

> Add an option to trigger memory writeback to sync given memory region
> with the corresponding backing store, case one is available.
> This extends the support for persistent memory, allowing syncing on-demand.
>
> Also, adding verification for msync support on host.
>
> Signed-off-by: Beata Michalska <address@hidden>
> ---
>  configure               | 24 ++++++++++++++++++++++++
>  exec.c                  | 38 ++++++++++++++++++++++++++++++++++++++
>  include/exec/memory.h   |  6 ++++++
>  include/exec/ram_addr.h |  6 ++++++
>  memory.c                | 12 ++++++++++++
>  5 files changed, 86 insertions(+)
>
> diff --git a/configure b/configure
> index 95134c0180..bdb7dc47e9 100755
> --- a/configure
> +++ b/configure
> @@ -5081,6 +5081,26 @@ if compile_prog "" "" ; then
>      fdatasync=yes
>  fi
>
> +##########################################
> +# verify support for msyc
> +
> +msync=no
> +cat > $TMPC << EOF
> +#include <unistd.h>
> +#include <sys/mman.h>
> +int main(void) {
> +#if defined(_POSIX_MAPPED_FILES) && _POSIX_MAPPED_FILES > 0 \
> +&& defined(_POSIX_SYNCHRONIZED_IO) && _POSIX_SYNCHRONIZED_IO > 0
> +return msync(NULL,0, MS_SYNC);
> +#else
> +#error Not supported
> +#endif
> +}
> +EOF
> +if compile_prog "" "" ; then
> +    msync=yes
> +fi
> +
>  ##########################################
>  # check if we have madvise
>
> @@ -6413,6 +6433,7 @@ echo "fdt support       $fdt"
>  echo "membarrier        $membarrier"
>  echo "preadv support    $preadv"
>  echo "fdatasync         $fdatasync"
> +echo "msync             $msync"
>  echo "madvise           $madvise"
>  echo "posix_madvise     $posix_madvise"
>  echo "posix_memalign    $posix_memalign"
> @@ -6952,6 +6973,9 @@ fi
>  if test "$fdatasync" = "yes" ; then
>    echo "CONFIG_FDATASYNC=y" >> $config_host_mak
>  fi
> +if test "$msync" = "yes" ; then
> +    echo "CONFIG_MSYNC=y" >> $config_host_mak
> +fi

I think it's best to split this configure check into a new prequel patch and...

>  if test "$madvise" = "yes" ; then
>    echo "CONFIG_MADVISE=y" >> $config_host_mak
>  fi
> diff --git a/exec.c b/exec.c
> index 235d6bc883..5ed6908368 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -65,6 +65,8 @@
>  #include "exec/ram_addr.h"
>  #include "exec/log.h"
>
> +#include "qemu/pmem.h"
> +
>  #include "migration/vmstate.h"
>
>  #include "qemu/range.h"
> @@ -2182,6 +2184,42 @@ int qemu_ram_resize(RAMBlock *block, ram_addr_t 
> newsize, Error **errp)
>      return 0;
>  }
>
> +/*
> + * Trigger sync on the given ram block for range [start, start + length]
> + * with the backing store if available.
> + * @Note: this is supposed to be a SYNC op.
> + */
> +void qemu_ram_writeback(RAMBlock *block, ram_addr_t start, ram_addr_t length)
> +{
> +    void *addr = ramblock_ptr(block, start);
> +
> +    /*
> +     * The requested range might spread up to the very end of the block
> +     */
> +    if ((start + length) > block->used_length) {
> +        error_report("%s: sync range outside the block boundires: "
> +                     "start: " RAM_ADDR_FMT " length: " RAM_ADDR_FMT
> +                     " block length: " RAM_ADDR_FMT " Narrowing down ..." ,
> +                     __func__, start, length, block->used_length);

Is this an error or just logging? error_report should be used for stuff
that the user needs to know about so it will appear on the HMP console
(or via stderr). If so what is the user expected to do? Have they
misconfigured their system?

> +        length = block->used_length - start;
> +    }
> +
> +#ifdef CONFIG_LIBPMEM
> +    /* The lack of support for pmem should not block the sync */
> +    if (ramblock_is_pmem(block)) {
> +        pmem_persist(addr, length);
> +    } else
> +#endif
> +    if (block->fd >= 0) {
> +#ifdef CONFIG_MSYNC
> +        msync((void *)((uintptr_t)addr & qemu_host_page_mask),
> +               HOST_PAGE_ALIGN(length), MS_SYNC);
> +#else
> +        qemu_fdatasync(block->fd);
> +#endif

... hide the implementation details in util/cutils.c, maybe as
qemu_msync()?

> +    }
> +}
> +
>  /* Called with ram_list.mutex held */
>  static void dirty_memory_extend(ram_addr_t old_ram_size,
>                                  ram_addr_t new_ram_size)
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index 2dd810259d..ff0d7937cf 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -1242,6 +1242,12 @@ void *memory_region_get_ram_ptr(MemoryRegion *mr);
>   */
>  void memory_region_ram_resize(MemoryRegion *mr, ram_addr_t newsize,
>                                Error **errp);
> +/**
> + * memory_region_do_writeback: Trigger writeback for selected address range
> + * [addr, addr + size]
> + *
> + */
> +void memory_region_do_writeback(MemoryRegion *mr, hwaddr addr, hwaddr size);
>
>  /**
>   * memory_region_set_log: Turn dirty logging on or off for a region.
> diff --git a/include/exec/ram_addr.h b/include/exec/ram_addr.h
> index a327a80cfe..d4bce81a03 100644
> --- a/include/exec/ram_addr.h
> +++ b/include/exec/ram_addr.h
> @@ -180,6 +180,12 @@ void qemu_ram_free(RAMBlock *block);
>
>  int qemu_ram_resize(RAMBlock *block, ram_addr_t newsize, Error **errp);
>
> +void qemu_ram_writeback(RAMBlock *block, ram_addr_t start, ram_addr_t 
> length);
> +
> +/* Clear whole block of mem */
> +#define qemu_ram_block_writeback(rb)    \
> +    qemu_ram_writeback(rb, 0, rb->used_length)
> +
>  #define DIRTY_CLIENTS_ALL     ((1 << DIRTY_MEMORY_NUM) - 1)
>  #define DIRTY_CLIENTS_NOCODE  (DIRTY_CLIENTS_ALL & ~(1 << DIRTY_MEMORY_CODE))
>
> diff --git a/memory.c b/memory.c
> index 61a254c3f9..436eb64737 100644
> --- a/memory.c
> +++ b/memory.c
> @@ -2228,6 +2228,18 @@ void memory_region_ram_resize(MemoryRegion *mr, 
> ram_addr_t newsize, Error **errp
>      qemu_ram_resize(mr->ram_block, newsize, errp);
>  }
>
> +
> +void memory_region_do_writeback(MemoryRegion *mr, hwaddr addr, hwaddr size)
> +{
> +    /*
> +     * Might be extended case needed to cover
> +     * different types of memory regions
> +     */
> +    if (mr->ram_block && mr->dirty_log_mask) {
> +        qemu_ram_writeback(mr->ram_block, addr, size);
> +    }
> +}
> +
>  /*
>   * Call proper memory listeners about the change on the newly
>   * added/removed CoalescedMemoryRange.


--
Alex Bennée