On Sat, Oct 03, 2020 at 05:40:58AM +0000, Graf (AWS), Alexander wrote:
> Hey Kevin,
>
> > Am 03.10.2020 um 03:47 schrieb Kevin O'Connor <ke...@koconnor.net>:
> >
> >
> >> 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.
>
> This is tail recursion, which any reasonable compiler converts into a jmp :).
> Take a look at the .o file - for me it did become a plain jump.
>
Okay, that's fine then.
> >
> > Otherwise, the patch series looks okay to me. (I don't have enough
> > knowledge of the nvme code to give a full review though).
>
> Thanks :)
I'll leave a few more days to allow others to comment, but otherwise
I'm fine with committing.
Cheers,
-Kevin
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