Re: [Qemu-devel] [PATCH v2 2/2] postcopy: Check for shared memory

[Halil Pasic <pasic@linux.vnet.ibm.com>]

On 03/09/2017 02:22 PM, Dr. David Alan Gilbert (git) wrote:
> From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> 
> Postcopy doesn't support migration of RAM shared with another process
> yet (we've got a bunch of things to understand).
> Check for the case and don't allow postcopy to be enabled.
> 
> Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> ---
>  migration/postcopy-ram.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
> 
> diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
> index effbeb6..dc80dbb 100644
> --- a/migration/postcopy-ram.c
> +++ b/migration/postcopy-ram.c
> @@ -95,6 +95,19 @@ static bool ufd_version_check(int ufd)
>      return true;
>  }
> 
> +/* Callback from postcopy_ram_supported_by_host block iterator.
> + */
> +static int test_range_shared(const char *block_name, void *host_addr,
> +                             ram_addr_t offset, ram_addr_t length, void *opaque)
> +{
> +    if (qemu_ram_is_shared(qemu_ram_block_by_name(block_name))) {
> +        error_report("Postcopy on shared RAM (%s) is not yet supported",
> +                     block_name);
> +        return 1;
> +    }
> +    return 0;
> +}
> +

Hm, this stuff with the iterator seemed a bit strange (too complicated)
first, but I'm not familiar with this code. I have no idea why is
RAMBlockIterFunc
 
typedef int (RAMBlockIterFunc)(const char *block_name, void *host_addr,
    ram_addr_t offset, ram_addr_t length, void *opaque)

and not 

typedef int (RAMBlockIterFunc)(RAMBlock *block, void *opaque).

The reason does not seem to be abstraction.

>  /*
>   * Note: This has the side effect of munlock'ing all of RAM, that's
>   * normally fine since if the postcopy succeeds it gets turned back on at the
> @@ -127,6 +140,11 @@ bool postcopy_ram_supported_by_host(void)
>          goto out;
>      }
> 
> +    /* We don't support postcopy with shared RAM yet */
> +    if (qemu_ram_foreach_block(test_range_shared, NULL)) {
> +        goto out;
> +    }
> +

But using ram_list directly does not seem to be a good alternative to me,
and I do not see a third alternative.

So besides some cosmetic stuff I have nothing to add. Cosmetic stuff is:
* why range instead of block in test_range_shared
* I think we could move this up so that we can return directly
and do not acquire resources which need cleanup

Regardless of the cosmetics:
Reviewed-by: Halil Pasic <pasic@linux.vnet.ibm.com>


>      /*
>       * userfault and mlock don't go together; we'll put it back later if
>       * it was enabled.
>