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Re: [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic A
From: |
Christoph Hellwig |
Subject: |
Re: [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic API |
Date: |
Sat, 14 Mar 2009 15:33:27 +0100 |
User-agent: |
Mutt/1.3.28i |
On Thu, Mar 12, 2009 at 02:57:09PM +0200, Avi Kivity wrote:
> When a scsi device is backed by a scsi generic device instead of an
> ordinary host block device, the block API is abused in a couple of annoying
> ways:
>
> - nb_sectors is negative, and specifies a byte count instead of a sector
> count
> - offset is ignored, since scsi-generic is essentially a packet protocol
>
> This overloading makes hacking the block layer difficult. Remove it by
> introducing a new explicit API for scsi-generic devices. The new API
> is still backed by the old implementation, but at least the users are
> insulated.
Getting rid of this is good, I found tons of issues in that area
with the I/O path exerciser I wrote last week. Hower the way the new
API is designed somewhat gets in the way of my patch series to support
Gerd's native preadv/pwritev.
One thing I wonder is why we go through the block layer at all for SG.
It is actually backed by a char device that doesn't have semantics like
a block device at all, given that it can't be seekend and is accessed
at byte granularity. We could just go directly to the native Linus
syscalls from scsi-generic.c (or a scsi-generic-linux.c if we want
some level of abstraction). I'll cook up a patch trying that once I'm
back home.
- [Qemu-devel] [PATCH 0/3] Remove ->bdrv_pread() internal block layer API (v2), Avi Kivity, 2009/03/12
- [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic API, Avi Kivity, 2009/03/12
- Re: [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic API,
Christoph Hellwig <=
- Re: [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic API, Avi Kivity, 2009/03/15
- Re: [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic API, Christoph Hellwig, 2009/03/15
- Re: [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic API, Avi Kivity, 2009/03/15
- Re: [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic API, Christoph Hellwig, 2009/03/15
- Re: [Qemu-devel] [PATCH 1/3] Add specialized block driver scsi generic API, Jamie Lokier, 2009/03/16
[Qemu-devel] [PATCH 3/3] Drop internal bdrv_pread()/bdrv_pwrite() APIs, Avi Kivity, 2009/03/12
[Qemu-devel] [PATCH 2/3] Add internal scsi generic block API, Avi Kivity, 2009/03/12
[Qemu-devel] Re: [PATCH 0/3] Remove ->bdrv_pread() internal block layer API (v2), Anthony Liguori, 2009/03/12