Re: [PATCH v2 3/3] hw/ufs: Support for UFS logical unit

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Jun 19, 2023 at 08:16:27PM +0900, Jeuk Kim wrote:
> On Fri, Jun 19, 2023, Stefan Hajnoczi wrote:
> >On Fri, Jun 16, 2023 at 03:58:27PM +0900, Jeuk Kim wrote:
> >> This commit adds support for ufs logical unit.
> >> The LU handles processing for the SCSI command,
> >> unit descriptor query request.
> >> 
> >> This commit enables the UFS device to process
> >> IO requests.
> >
> >Is UFS a SCSI Host Bus Adapter capable of exposing any SCSI device? The
> >code is written as if UFS was a special-purpose SCSI bus that cannot
> >handle regular SCSI devices already emulated by QEMU (like scsi-hd). As
> >a result, it duplicates a lot of SCSI device code instead of just
> >focussing on unwrapping/wrapping the SCSI commands and responses from
> >the UFS interface.
> >
> >Would it be possible to have:
> >
> >  --device ufs,id=<bus-name>
> >  --device scsi-hd,bus=<bus-name>
> >
> >?
> >
> >I think that would involve less code and be more flexible.
> >
> 
> Unfortunately, UFS is not a generic SCSI Host Bus Adapter.
> UFS uses the SCSI specification to communicate with the driver,
> but its behaviour is very different from that of a typical SCSI device.
> (So it's intentional that UFS looks like a special-purpose SCSI bus.)
> 
> For example, UFS has the well-known lu.
> Unlike typical SCSI devices, where each lu is independent,
> UFS can control other lu's through the well-known lu.
> 
> Therefore, UFS can only work properly with ufs-lu, and not with
> other scsi devices such as scsi-hd. :'(
> 
> That's why I made the UFS bus and added the ufs_bus_check_address()
> to prevent normal scsi devices and UFS from connecting to each other.
> 
> Also, in the future, I will add more ufs-specific features
> like hibernation and zoned, which are different from normal SCSI devices.
> 
> So personally, I think we should define ufs-lu separately as we do now.
> Is that okay?

Yes, I think that makes sense. Thanks for explaining.

Paolo Bonzini is the QEMU SCSI emulation maintainer. He might have more
thoughts about this. I have CCed him, but I think you can continue with
the current approach unless Paolo decides to get involved in this patch
series.

Stefan

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