Re: [PATCH RFC v2 4/8] hw/block/nvme: try to deal with the iov/qsg duality

From: Keith Busch <kbusch@kernel.org>
To: Klaus Jensen <its@irrelevant.dk>
Cc: Kevin Wolf <kwolf@redhat.com>, Fam Zheng <fam@euphon.net>,
	qemu-block@nongnu.org, Klaus Jensen <k.jensen@samsung.com>,
	Gollu Appalanaidu <anaidu.gollu@samsung.com>,
	qemu-devel@nongnu.org, Max Reitz <mreitz@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>
Subject: Re: [PATCH RFC v2 4/8] hw/block/nvme: try to deal with the iov/qsg duality
Date: Tue, 9 Feb 2021 00:45:15 +0900	[thread overview]
Message-ID: <20210208154515.GC16360@redsun51.ssa.fujisawa.hgst.com> (raw)
In-Reply-To: <20210207214940.281889-5-its@irrelevant.dk>

On Sun, Feb 07, 2021 at 10:49:36PM +0100, Klaus Jensen wrote:
> From: Klaus Jensen <k.jensen@samsung.com>
> 
> Introduce NvmeSg and try to deal with that pesky qsg/iov duality that
> haunts all the memory-related functions.
> 
> Signed-off-by: Klaus Jensen <k.jensen@samsung.com>
> ---
>  hw/block/nvme.h |   8 ++-
>  hw/block/nvme.c | 171 ++++++++++++++++++++++++------------------------
>  2 files changed, 90 insertions(+), 89 deletions(-)
> 
> diff --git a/hw/block/nvme.h b/hw/block/nvme.h
> index cb2b5175f1a1..0e4fbd6990ad 100644
> --- a/hw/block/nvme.h
> +++ b/hw/block/nvme.h
> @@ -29,6 +29,11 @@ typedef struct NvmeAsyncEvent {
>      NvmeAerResult result;
>  } NvmeAsyncEvent;
>  
> +typedef struct NvmeSg {
> +    QEMUSGList   qsg;
> +    QEMUIOVector iov;
> +} NvmeSg;

It's always one or the other, right? If so, this could be a 'union'
type, and then you'd just need an indicator field to say which type it
is. There may be a meaninful memory savings that way since we
potentially allocate thousands of these.