Re: [Qemu-devel] On block interface types in general, IF_AHCI in particular

[Jason Baron <jbaron@redhat.com>]

On Tue, Oct 30, 2012 at 03:43:20PM +0100, Markus Armbruster wrote:
> The primary purpose of this memo is a brain dump on how block interface
> types are used, and what that means for AHCI.  A secondary purpose is to
> disabuse Alex of the notion that -drive is simple ;)
> 
> 
> BlockInterfaceType really just serves -drive and its monitor cousins
> drive_add and pci_add[*].
> 
> Note: we have a whole zoo of convenience options to define drives, but
> they're all just shorthand for -drive, so let's ignore them here.
> 
> 
> Let's start with -drive.  -drive creates a block backend, and leaves it
> in a place where board code can find it if it cares.
> 
> That place is indexed by a triple (BlockInterfaceType type, int bus, int
> unit), corresponding to -drive options if, bus, unit.
> 
> The actual work is done by drive_init().  Its opts argument comes
> straight from -drive's argument.
> 
> If option if is missing, type is set to a board-specific default.
> 
> If option bus is missing, bus is set to zero.
> 
> If option unit is missing, the system picks the first unused bus, unit
> starting with (bus, 0).
> 
> For type IF_IDE, unit must be 0 or 1.
> 
> For type IF_SCSI, unit must be 0..6.  Note that the range is fine for
> 8-bit SCSI, and inappropriate for anything else.
> 
> Complication: option index.  This is an alternate way to specify bus and
> unit.
> 
> For type IF_IDE, bus = index / 2, unit = index % 2.
> 
> For type IF_SCSI, bus = index / 7, unit = index % 7.  Again, fine for
> 8-bit SCSI, not fine for everything else.
> 
> For any other type, bus = 0, unit = index.  There's no way to specify
> bus > 0 with index.
> 
> This mapping from index to (bus, unit) is ABI.
> 
> (type, bus, unit) also get put into the drive ID when the user doesn't
> specify one (again ABI), but let's ignore that here.
> 
> 
> Now examine what board code does with (type, bus, unit) triples.  In
> general, board code looks up the triples it knows, and it finds a
> backend, it creates a suitable device model connected to the it.
> 
> Same thing, different perspective: a board has a fixed set of onboard
> devices.  They may be associated with a well-known number of (type, bus,
> unit) triples.  The association is ABI.
> 
> Example: "connex" associates (IF_PFLASH, 0, 0) with its CFI parallel
> flash.  It errors out when the triple isn't found.
> 
> Example: "g3beige" associates its PMAC IDE's master with (IF_IDE, 0, 0),
> the slave with (IF_IDE, 0, 1), its CMD646's primary master with (IF_IDE,
> 1, 0), its slave with (IF_IDE, 1, 1).  A suitable IDE device is created
> when a triple is found.  It doesn't associate its secondary master and
> slave with any triple (which means you can't put drives there with
> -drive if=ide).
> 
> Boards never associate (IF_NONE, ...) with anything.  The backends
> behind these tripes are for use with -device.
> 
> Some boards create additional device models when they find certain
> triples.  What gets created when is ABI.
> 
> Example: "pc" finds the maximum bus N that occurs in any (IF_SCSI, ...)
> triple, then creates N+1 lsi53c895a SCSI HBAs, and connects all
> (IF_SCSI, B, U) to a suitable SCSI device model with SCSI ID U on the
> B-th HBA.
> 
> Example: "spapr" does the same, except it creates spapr-vscsi HBAs.
> 
> Boards generally ignore triples they don't associate with anything
> silently, but there are exceptions.
> 
> Example: "sun4m" errors out if it finds (IF_SCSI, B, U) with B>0.
> 
> Ignored triples can still be used with -device.
> 
> Example: "pc" silently ignores the (IF_SD, ...), but these drives are
> still usable with -device ide-cd,drive=sd0 and such.  Filed under
> "stupid stunts".  It's ABI all the same.
> 
> There's a twist for (IF_SCSI, B, U) triples ignored by the board:
> creating the B-th a SCSI HBA with -device also creates a suitable SCSI
> device model with SCSI ID U on that HBA.
> 
> Example: "sun4u" doesn't provide a SCSI HBA, and normally ignores
> (IF_SCSI, 0, 0) silently.  But if you then create a SCSI HBA with
> -device, this also creates a suitable SCSI device model with SCSI ID 0
> on that HBA.
> 
> Boards can associate block interface types with whatever device models
> they choose, even totally different kinds than the name of the block
> interface type suggests.  You may call that abuse.  I call it ABI.
> 
> Example: "s390-virtio" creates virtio-blk-s390 device models for
> (IF_IDE, ...) triples.
> 
> Some boards may do weird shit not covered here.  What weird shit gets
> done when is ABI.
> 
> The important lesson here is that the meaning of -drive parameters if,
> bus, unit and index depends entirely on the board (except for -drive
> if=none, of course), and is part of the board's ABI.
> 
> 
> Next are drive_add and pci_add.  These commands are quite limited in
> what they can do.
> 
> drive_add "" if=none,OPTS... works just like -drive if=none,OPTS...: it
> creates a block backend for use with device_add.
> 
> drive_add ADDR if=scsi,OPTS... resembles -drive if=scsi,OPTS..., except
> it doesn't leave device model creation to the board code: it creates a
> suitable SCSI device connected to the SCSI HBA with PCI address ADDR.
> 
> pci_add ADDR storage if=scsi,OPTS additionally creates an lsi53c895a HBA
> with PCI address ADDR.  Even when the board creates some other HBA for
> -drive if=scsi.
> 
> pci_add ADDR storage if=virtio,OPTS creates a virtio-blk-pci device with
> PCI address ADDR, similar to -drive if=virtio,addr=ADDR,OPTS.
> 
> The only general way to hot plug a block device is drive_add with
> if=none, then device_add.  The other forms cover only special cases.
> Generalizing them involves making them work like -drive: leave device
> model creation to board code.  Which had to be added to every board
> supporting hot plug.  Hardly worth the trouble.
> 
> 
> What does this all mean for AHCI?
> 
> The argument that we must have IF_AHCI because AHCI needs different
> guest OS drivers than IDE doesn't hold water.  Plenty of precedence
> above: IF_IDE can get you an ide-hd connected to some IDE controller, or
> a virtio-blk-s390 device, and IF_SCSI can get you a scsi-hd connected to
> totally different SCSI HBAs.  I can't see why IF_IDE giving you an
> ide-hd connected to ich9-ahci on q35 would be any different.
> 
> There's a related argument that I find more compelling: we may want
> if=ahci to let users choose nicely between IDE and AHCI.  Makes sense
> only if we have boards providing both kind of controllers onboard.  q35
> doesn't.
> 

This was my primary argument for ahci. However, I didn't realize
initially that if=blah was only applied for the default controller. IE
it doesn't spawn controllers if they don't exist. If we are limiting
if=blah to attaching to the default controller, now and in the future, I
think the argument for IF_AHCI becomes less compelling. 

> Another argument for IF_AHCI is about the permissible range of unit and
> the mapping of index to bus and unit, both detemined by the maximum
> permissible unit number.  Right now, the maximum only depends on the
> block interface type, not the board, and IF_IDE's maximum 2 is
> inappropriate for AHCI (ich9-ahci wants 6, assuming we model it as a
> single bus for purposes of -drive).  However, the same problem already
> exists for IF_SCSI: its maximum 7 is inappropriate for anything but
> crusty old 8-bit SCSI.  Creating IF_AHCI bypasses the problem with
> IF_IDE's maximum, but leaves the IF_SCSI problem unsolved.  Hmm.
> 
> How do we plan to handle the next member of the ATA controller family?
> Create yet another block interface type for it?
> 

The way IF_AHCI is implemented, the board controlls mapping of index to
bus and unit. So I don't think we would need to create a new one.

> I'm not saying IF_AHCI is wrong.  I'm just saying some of the arguments
> for it aren't compelling.
> 
> 
> [*] There are two other uses of drive_init(), in hw/pc_sysfw.c and
> hw/usb/dev_storage.  They both create both backend and frontend, thus
> the BlockInterfaceType they use doesn't really matter.

As you know, I'm trying to come to consensus on this in the interest of getting
q35 into 1.3.

I just prototyped a patch that removes IF_AHCI. And models the 6 port
ahci controller as 3 busses with 2 units per bus. This should allow all
existing if=ide usages to continue to work as expected. Is that more
palatable?

Thanks,

-Jason