Re: [Qemu-devel] [RFC 05/29] postcopy: use UFFDIO_ZEROPAGE only when available

[Peter Xu]

On Wed, Jun 28, 2017 at 08:00:23PM +0100, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <address@hidden>
> 
> Use the recently added migration flag to hold whether
> each RAMBlock has the UFFDIO_ZEROPAGE capability, use it
> when it's available.
> 
> This allows the use of postcopy on tmpfs as well as hugepage
> backed files.
> 
> Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> ---
>  migration/migration.h    |  4 ++++
>  migration/postcopy-ram.c | 12 +++++++++---
>  2 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/migration/migration.h b/migration/migration.h
> index d9a268a3af..d109635d08 100644
> --- a/migration/migration.h
> +++ b/migration/migration.h
> @@ -20,6 +20,10 @@
>  #include "exec/cpu-common.h"
>  #include "qemu/coroutine_int.h"
>  
> +/* Migration flags to be set using qemu_ram_set_migration_flags */
> +/* Postcopy can atomically zero pages in this RAMBlock */
> +#define QEMU_MIGFLAG_POSTCOPY_ZERO   0x00000001
> +
>  /* State for the incoming migration */
>  struct MigrationIncomingState {
>      QEMUFile *from_src_file;
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index be2a8f8e02..96338a8070 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -408,6 +408,12 @@ static int ram_block_enable_notify(const char 
> *block_name, void *host_addr,
>          error_report("%s userfault: Region doesn't support COPY", __func__);
>          return -1;
>      }
> +    if (reg_struct.ioctls & ((__u64)1 << _UFFDIO_ZEROPAGE)) {
> +        RAMBlock *rb = qemu_ram_block_by_name(block_name);
> +        qemu_ram_set_migration_flags(rb, qemu_ram_get_migration_flags(rb) |
> +                                         QEMU_MIGFLAG_POSTCOPY_ZERO);

Shall we use atomic_or() inside qemu_ram_set_migration_flags()? Then
no need to fetch, and we'll be thread safe as well?

> +    }
> +
>  
>      return 0;
>  }
> @@ -620,11 +626,11 @@ int postcopy_place_page(MigrationIncomingState *mis, 
> void *host, void *from,
>  int postcopy_place_page_zero(MigrationIncomingState *mis, void *host,
>                               RAMBlock *rb)
>  {
> +    size_t pagesize = qemu_ram_pagesize(rb);
>      trace_postcopy_place_page_zero(host);
>  
> -    if (qemu_ram_pagesize(rb) == getpagesize()) {
> -        if (qemu_ufd_copy_ioctl(mis->userfault_fd, host, NULL, getpagesize(),
> -                                rb)) {
> +    if (qemu_ram_get_migration_flags(rb) & QEMU_MIGFLAG_POSTCOPY_ZERO) {

IIUC, _UFFDIO_ZEROPAGE is not supported on huge pages. If so, would
here worth a comment?

> +        if (qemu_ufd_copy_ioctl(mis->userfault_fd, host, NULL, pagesize, 
> rb)) {
>              int e = errno;
>              error_report("%s: %s zero host: %p",
>                           __func__, strerror(e), host);
> -- 
> 2.13.0
> 

Thanks,

-- 
Peter Xu