On Wed, Sep 30, 2020 at 11:10:56PM +0200, Alexander Graf wrote:
> Some NVMe controllers only support small maximum request sizes, such as
> the AWS EBS NVMe implementation which only supports NVMe requests of up
> to 32 pages (256kb) at once.
>
> BIOS callers can exceed those request sizes by defining sector counts
> above this threshold. Currently we fall back to the bounce buffer
> implementation for those. This is slow.
>
> This patch introduces splitting logic to the NVMe I/O request code so
> that every NVMe I/O request gets handled in a chunk size that is
> consumable by the NVMe adapter, while maintaining the fast path PRPL
> logic we just introduced.
>
> Signed-off-by: Alexander Graf <g...@amazon.com>
> ---
> src/hw/nvme.c | 16 ++++++++++++++++
> 1 file changed, 16 insertions(+)
>
> diff --git a/src/hw/nvme.c b/src/hw/nvme.c
> index b92ca52..cc37bca 100644
> --- a/src/hw/nvme.c
> +++ b/src/hw/nvme.c
> @@ -727,6 +727,22 @@ nvme_cmd_readwrite(struct nvme_namespace *ns, struct
> disk_op_s *op, int write)
> u16 const max_blocks = NVME_PAGE_SIZE / ns->block_size;
> u16 i;
>
> + /* Split up requests that are larger than the device can handle */
> + if (op->count > ns->max_req_size) {
> + u16 count = op->count;
> +
> + /* Handle the first max_req_size elements */
> + op->count = ns->max_req_size;
> + if (nvme_cmd_readwrite(ns, op, write))
> + return res;
> +
> + /* Handle the remainder of the request */
> + op->count = count - ns->max_req_size;
> + op->lba += ns->max_req_size;
> + op->buf_fl += (ns->max_req_size * ns->block_size);
> + return nvme_cmd_readwrite(ns, op, write);
> + }
Depending on the disk access, this code could run with a small stack.
I would avoid recursion.
Otherwise, the patch series looks okay to me. (I don't have enough
knowledge of the nvme code to give a full review though.)
-Kevin
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