Re: [PATCH V1] migration: cpr breaks SNP guest

[Fabiano Rosas <farosas@suse.de>]

Steve Sistare <steven.sistare@oracle.com> writes:

> With aux-ram-share=off, booting an SNP guest fails with:
>
>   ../util/error.c:68: error_setv: Assertion `*errp == NULL' failed.
>
> This is because a CPR blocker for the guest_memfd ramblock is added
> twice, once in ram_block_add_cpr_blocker because aux-ram-share=off so
> rb->fd < 0, and once in ram_block_add for a specific guest_memfd blocker.
>
> To fix, add the guest_memfd blocker iff a generic one would not be
> added by ram_block_add_cpr_blocker.
>
> Fixes: 094a3dbc55df ("migration: ram block cpr blockers")
> Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
> Reported-by: Michael Roth <michael.roth@amd.com>
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> ---
>  system/physmem.c | 19 ++++++++++++-------
>  1 file changed, 12 insertions(+), 7 deletions(-)
>
> diff --git a/system/physmem.c b/system/physmem.c
> index e97de3e..cfafb06 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -1908,13 +1908,18 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
>              goto out_free;
>          }
>  
> -        error_setg(&new_block->cpr_blocker,
> -                   "Memory region %s uses guest_memfd, "
> -                   "which is not supported with CPR.",
> -                   memory_region_name(new_block->mr));
> -        migrate_add_blocker_modes(&new_block->cpr_blocker, errp,
> -                                  MIG_MODE_CPR_TRANSFER,
> -                                  -1);
> +        /*
> +         * Add a specific guest_memfd blocker if a generic one would not be
> +         * added by ram_block_add_cpr_blocker.
> +         */
> +        if (new_block->fd >= 0 && qemu_ram_is_shared(new_block)) {

Can we avoid having two instances of the same condition that will need
to be kept in sync? What about:

/*
 * Preserving guest_memfd may be sufficient for CPR, but it has not
 * been tested yet.
 */
if (ram_is_cpr_compatible(new_block)) {
...

> +            error_setg(&new_block->cpr_blocker,
> +                       "Memory region %s uses guest_memfd, "
> +                       "which is not supported with CPR.",
> +                       memory_region_name(new_block->mr));
> +            migrate_add_blocker_modes(&new_block->cpr_blocker, errp,
> +                                      MIG_MODE_CPR_TRANSFER, -1);
> +        }
>      }
>  
>      ram_size = (new_block->offset + new_block->max_length) >> TARGET_PAGE_BITS;