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Re: [Qemu-devel] [RFC 0/4] Adding -cdrom, -hd[abcd] and -drive file=...

From: John Snow
Subject: Re: [Qemu-devel] [RFC 0/4] Adding -cdrom, -hd[abcd] and -drive file=... to Q35
Date: Tue, 19 Aug 2014 14:59:39 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.7.0 

On 08/19/2014 02:08 PM, Markus Armbruster wrote:

John Snow <address@hidden> writes:


On 08/19/2014 04:05 AM, Markus Armbruster wrote:

John Snow <address@hidden> writes:


Currently, the drive definitions created by drive_new() when using
the -drive file=...[,if=ide] or -cdrom or -hd[abcd] options are not
picked up by the Q35 initialization routine.

To fix this, we have to add hooks to search for these drives using
something like pc_piix's ide_drive_get and then add them using
something like pci_ide_create_devs.


ide_drive_get() isn't pc_piix's, it's a helper function in the IDE core
which boards (not just pc_piix) use to find the if=ide drives.  It fills
in an array of DriveInfo.

pci_ide_create_devs() is a helper function in the IDE PCI code which PCI
IDE controllers (not just piix3-ide) use to create IDE devices for an
array of DriveInfo.


Yes. I meant to say pc_piix's /call to/ ide_drive_get. I would have to
patch up the other boards if I changed this function! Only an RFC
before I got too far down this path :]


Where it gets slightly wonky is the fact that if=ide is specified
to use two devices per bus, whereas AHCI does not utilize that
same master/slave mechanic. Therefore, many places inside of
blockdev.c where we add and define new drives use incorrect math
for AHCI devices and try to place them on impossible buses.
Notably -hdb and -hdd would become inaccessible.


Yes.


To remedy this, I added a new interface type, IF_AHCI. Corresponding
to this change, I modified the default machine properties for Q35
to use this interface as a default.

The changes appear to work well, but where I'd like some feedback
is what should happen if people do something like:

qemu -M q35 -drive if=ide,file=fedora.qcow2

The code as presented here is not going to look for or attempt to
connect IDE devices, because it is now looking for /AHCI/ devices.

At worst, this may break a few existing scripts, but I actually think
that since the if=ide,file=... shorthand never worked to begin with,
the impact might actually be minimal.

But since the legacy IDE interface of the ICH9 is as of yet unemulated,
the if=ide drives don't have a reasonable place to go yet. I am also
not sure what a reasonable way to handle people specifying BOTH
if=ide and if=ahci drives would be.


We've been through IF_AHCI before, more than once, but that was before
you got involved :)

The problem at hand is that "-drive if=ide" and its sugared forms -hda,
-hdb, -cdrom, ... don't work with Q35.

You provide a solution for the sugared forms, you leave the problem
unsolved for the unsugared form, and you add a new problem: -drive
if=ahci doesn't work with i440FX.  Progress, sort of.


Adding a call to boards that support the AHCI device during their
initialization should be easy enough, if we decide that "ide means
ISA/PCI, ahci means the AHCI HBA." I could probably even write one
generic routine between i440fx and q35 to do both IDE/AHCI.

If we decide that IF_IDE and IF_AHCI mean different things, the
problem of the unsugared form being unsolved depends on me (well, or
someone) implementing the legacy IDE interface for Q35.


Let me come back to this further down.


Let's take a step back, and recap previous discussion.  There are two
defensible point of views, in my opinion.

One is that IDE and AHCI should be separate interface types, just like
IDE and SCSI are.

Attempts to define an if=X drive with a board that doesn't provide a
controller for X should fail[*].  Only onboard controllers matter,
add-ons plugged in with -device don't.  An i440FX board provides only
IDE.  A Q35 board provides only AHCI, not IDE.  If we implement an
ich9-ahci legacy mode, and switch it on, then it provides only IDE, not
AHCI.  Or maybe both, depending on how we do it.


I think I am leaning towards this viewpoint, but it depends on what
"interface" means in QEMU. Currently, the number of units per bus is
tied to the "interface" and clearly the AHCI SATA interface only
supports one per bus, so semantically this makes sense.


An index <-> (bus, unit) mapping doesn't make an interface!  Yes, it's
tightly coupled to the interface, but that became wrong way back when we
went beyond 8-bit SCSI HBAs, long before we added up AHCI HBAs.
"Interface" seems nebulous in QEMU. In the physical world, there definitely is a difference between IDE/EIDE/PATA and SATA/AHCI. Different physical and link layers -- the verbs stayed the same. What I am getting at is that I am not sure what an "interface" is /supposed/ to encompass here in QEMU. Underlying device type? If that's the case, then IDE/AHCI are definitely identical. 


I think the real ICH9 AHCI device supports only fully AHCI or fully
legacy, but the AHCI spec itself appears to allow you to run a
mixed-mode device.

I am not sure we have a usage case for mixed-mode, so enforcing
either/or for the AHCI device makes sense for now, I think.


I can't see a use for mixed mode, either.


The other point of view is that IDE and AHCI are no more different than
the different kinds of SCSI HBAs.  This is certainly true from a qdev
point of view: just like SCSI devices can connect to any SCSI qbus,
regardless of the HBA providing it, so can IDE devices connect to any
IDE qbus, regardless of the controller providing it.


Yes... Really the only difference are some mapping semantics. I don't
think there's any /other/ reason I needed IF_AHCI, of course, I wasn't
around for the previous discussions, so maybe there are other reasons
I am not aware of.


I've always been in the "we don't need or want if=ahci" camp :)

The one argument for if=ahci I found convincing was a desire for Q35
with its ICH9 in legacy mode.  And that's just as easily done with a
machine option.  Personally, I find that more natural.
So piix would always try to add to the PCI IDE bus, and Q35 would always try to add to the AHCI bus.
With a machine option for Q35, we could tell it to use the AHCI device in legacy mode and add to that device accordingly.
Do we care about the case for adding an AHCI device to piix? Do we change the behavior of what bus we use by a machine option (like a default_hba machine property and hba_type drive properties?), or by the presence of an AHCI device if someone adds one?
As it stands now, using if=ahci or if=ide is a pretty strong hint for what bus to look for and add to; though it is inflexible between command invocations that use different HBAs. 


There's a wrinkle: the mapping between index to (bus, unit).  This
mapping is ABI.  The current mapping makes sense for the first
generation of controllers: PATA (two devices per bus, thus
if_max_devs[IF_IDE] = 2), and 8-bit SCSI (seven per bus, thus
if_max_devs[IF_SCSI = 7).

The mapping is silly for newer SCSI HBAs.  Commit 622b520f tried to make
it less silly, but had to be reverted in 27d6bf4 because the silliness
was ABI.

The mapping is also silly for ich9-ahci.  You side-step that silliness
only, by adding a new interface type for it.  But shouldn't we add a
number of SCSI interface types then, too?  Where does that end?

Can we do better?  I think we can, by making this part of the ABI
board-specific.  The general form of the mapping remains

      (bus, unit) = (index / N, index % N)

but N now depends on board and interface type, not just the latter.

If the board connects if=scsi to an lsi53c895a, then N = 7.

If the board connects if=ide to an piix3-ide, then N = 2.

If the board connects if=ide to an ich9-ahci, then N = 1.

I trust you get the idea :)


I suppose we could make it something like:
if (HBA.max_units > 0) {
   N := min(HBA.max_units, IF.max_units);
} else {
   N := IF.max_units;
}
(bus, unit) := (index / N, index % N);

Which sets a default property for the interface but allows the device
(not the board) to override. Does that make more sense? If we allow
people to wire up an AHCI device to piix, we'll run back into the same
problems of the bus/unit mappings unless we make this a device
property.


Yes, it is a property of the device (property not in the qdev sense).

Why would HBA.max_units ever be greater than IF.max_units?

If the answer is "only if somebody screwed up the HBA device model",
then the above can be simplified to just N = HBA.max_units.
Oh, yeah. I did Web UI programming for a while during college. Not trusting plugin values is a side effect! I might pepper in an assertion to make it clear that you can't bump the number of units /up/, though. That's a smarter thing to do. 


I do feel like I'd rather just make it an interface property and have
people specify which type of bus they want to wire it up to, but that
does create a lot of disparity against the SCSI devices.


What do you mean by "interface property"?
A logical property of the interface specification; what QEMU and this patch does now. 


[*] Currently, they're silently ignored with most boards for most X, but
I regard that as implementation defect.


Yes. Is there a bool in the drive info array that we can set to say
"this drive has been added as a device" and check for any that went
unset?


Not yet :)


        I can add one and a routine to check for it, which may help
flush out more of the weird legacy sugar option bugs.


We do something like that for -netdev:

     $ qemu -nodefaults -display none -netdev user,id=foo
     Warning: netdev foo has no peer

and -net:

     $ qemu -nodefaults -display none -net user,id=foo
     Warning: vlan 0 with no nics

I think as long as we leave picking up configuration to boards, having
the boards mark the pieces they pick up is the best we can do.

An alternative to leaving it to boards is making the boads define
callbacks that get fed configuration.  But that's more surgery.

There's more than just -netdev, -net and -drive, though.  Many command
line options to configure devices also merely create a piece of
configuration for boards to pick up:

* Character devices (-serial, -parallel) end up in serial_hds[],
   parallel_hds[].

* Graphics devices (-vga) end up in vga_interface_type.

These all need the same "did the board pick it up?" check as -drive.

Some old options have been converted to work independent of boards:

* Virtio consoles (-virtioconsole) in commit 98b1925.

* Sound devices (-soundhw) in commit b3e6d59.

Newer convenience options should always worked this way.  -watchdog
does.

I may have missed options.
That is indeed more than a few. I probably need a little bit more exposure to different boards and how configuration works before I attack the problem as a whole. I may just fix -drive for now ... 




(Sugar attracts bugs. heh-heh-heh...)


Indeed!
Your position is pretty clear. I will give other people the time to chime in before I do too much more work on this, though I will probably go ahead and add the unused drive check now, since we'll want that no matter which path we take. 

--
—js
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