Re: [PATCH 04/17] block/nvme: Be explicit we share NvmeIdCtrl / NvmeIdNs structures

[Stefan Hajnoczi]

On Thu, Jun 25, 2020 at 08:48:25PM +0200, Philippe Mathieu-Daudé wrote:
> We allocate an unique chunk of memory then use it for two
> different structures. Introduce the 'idsz_max' variable to
> hold the maximum size, to make it clearer the size is enough
> to hold the two structures.
> 
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
> FIXME: reword with something that makes more sense...
> ---
>  block/nvme.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/block/nvme.c b/block/nvme.c
> index 71f8cf27a8..ffda804a8e 100644
> --- a/block/nvme.c
> +++ b/block/nvme.c
> @@ -438,6 +438,7 @@ static void nvme_identify(BlockDriverState *bs, int 
> namespace, Error **errp)
>      BDRVNVMeState *s = bs->opaque;
>      NvmeIdCtrl *idctrl;
>      NvmeIdNs *idns;
> +    size_t idsz_max;
>      NvmeLBAF *lbaf;
>      uint8_t *resp;
>      uint16_t oncs;
> @@ -448,14 +449,15 @@ static void nvme_identify(BlockDriverState *bs, int 
> namespace, Error **errp)
>          .cdw10 = cpu_to_le32(0x1),
>      };
>  
> -    resp = qemu_try_blockalign0(bs, sizeof(NvmeIdCtrl));
> +    idsz_max = MAX_CONST(sizeof(NvmeIdCtrl), sizeof(NvmeIdNs));
> +    resp = qemu_try_blockalign0(bs, idsz_max);
>      if (!resp) {
>          error_setg(errp, "Cannot allocate buffer for identify response");
>          goto out;
>      }
>      idctrl = (NvmeIdCtrl *)resp;
>      idns = (NvmeIdNs *)resp;
> -    r = qemu_vfio_dma_map(s->vfio, resp, sizeof(NvmeIdCtrl), true, &iova);
> +    r = qemu_vfio_dma_map(s->vfio, resp, idsz_max, true, &iova);

_nvme_check_size() has compile-time asserts that check
sizeof(NvmeIdCtrl) == sizeof(NvmeIdNs) == 4096.

I suggest the following cleanup:

  union {
      NvmeIdCtrl ctrl;
      NvmeIdNs ns;
  } *id;
  ...
  id = qemu_try_blockalign0(bs, sizeof(*id));
  ...
  r = qemu_vfio_dma_map(s->vfio, resp, sizeof(*id), true, &iova);

and accesses to idctl are replaced with id->ctrl and idns with id->ns.

This eliminates the casts, makes it clear that this data is overlapping,
and avoids the need for idsz_max.

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