Re: [Qemu-devel] [PATCH 24/58] PPC: E500: Add PV spinning code

[Alexander Graf]

On 27.09.2011, at 18:53, Blue Swirl wrote:

> On Tue, Sep 27, 2011 at 3:59 PM, Alexander Graf <address@hidden> wrote:
>> 
>> On 27.09.2011, at 17:50, Blue Swirl wrote:
>> 
>>> On Mon, Sep 26, 2011 at 11:19 PM, Scott Wood <address@hidden> wrote:
>>>> On 09/24/2011 05:00 AM, Alexander Graf wrote:
>>>>> On 24.09.2011, at 10:44, Blue Swirl wrote:
>>>>>> On Sat, Sep 24, 2011 at 8:03 AM, Alexander Graf <address@hidden> wrote:
>>>>>>> On 24.09.2011, at 09:41, Blue Swirl wrote:
>>>>>>>> On Mon, Sep 19, 2011 at 4:12 PM, Scott Wood <address@hidden> wrote:
>>>>>>>>> The goal with the spin table stuff, suboptimal as it is, was something
>>>>>>>>> that would work on any powerpc implementation.  Other
>>>>>>>>> implementation-specific release mechanisms are allowed, and are
>>>>>>>>> indicated by a property in the cpu node, but only if the loader knows
>>>>>>>>> that the OS supports it.
>>>>>>>>> 
>>>>>>>>>> IIUC the spec that includes these bits is not finalized yet. It is 
>>>>>>>>>> however in use on all u-boot versions for e500 that I'm aware of and 
>>>>>>>>>> the method Linux uses to bring up secondary CPUs.
>>>>>>>>> 
>>>>>>>>> It's in ePAPR 1.0, which has been out for a while now.  ePAPR 1.1 was
>>>>>>>>> just released which clarifies some things such as WIMG.
>>>>>>>>> 
>>>>>>>>>> Stuart / Scott, do you have any pointers to documentation where the 
>>>>>>>>>> spinning is explained?
>>>>>>>>> 
>>>>>>>>> https://www.power.org/resources/downloads/Power_ePAPR_APPROVED_v1.1.pdf
>>>>>>>> 
>>>>>>>> Chapter 5.5.2 describes the table. This is actually an interface
>>>>>>>> between OS and Open Firmware, obviously there can't be a real hardware
>>>>>>>> device that magically loads r3 etc.
>>>> 
>>>> Not Open Firmware, but rather an ePAPR-compliant loader.
>>> 
>>> 'boot program to client program interface definition'.
>>> 
>>>>>>>> The device method would break abstraction layers,
>>>> 
>>>> Which abstraction layers?
>>> 
>>> QEMU system emulation emulates hardware, not software. Hardware
>>> devices don't touch CPU registers.
>> 
>> The great part about this emulated device is that it's basically guest 
>> software running in host context. To the guest, it's not a device in the 
>> ordinary sense, such as vmport, but rather the same as software running on 
>> another core, just that the other core isn't running any software.
>> 
>> Sure, if you consider this a device, it does break abstraction layers. Just 
>> consider it as host running guest code, then it makes sense :).
>> 
>>> 
>>>>>>>> it's much like
>>>>>>>> vmport stuff in x86. Using a hypercall would be a small improvement.
>>>>>>>> Instead it should be possible to implement a small boot ROM which puts
>>>>>>>> the secondary CPUs into managed halt state without spinning, then the
>>>>>>>> boot CPU could send an IPI to a halted CPU to wake them up based on
>>>>>>>> the spin table, just like real HW would do.
>>>> 
>>>> The spin table, with no IPI or halt state, is what real HW does (or
>>>> rather, what software does on real HW) today.  It's ugly and inefficient
>>>> but it should work everywhere.  Anything else would be dependent on a
>>>> specific HW implementation.
>>> 
>>> Yes. Hardware doesn't ever implement the spin table.
>>> 
>>>>>>>> On Sparc32 OpenBIOS this
>>>>>>>> is something like a few lines of ASM on both sides.
>>>>>>> 
>>>>>>> That sounds pretty close to what I had implemented in v1. Back then the 
>>>>>>> only comment was to do it using this method from Scott.
>>>> 
>>>> I had some comments on the actual v1 implementation as well. :-)
>>>> 
>>>>>>> So we have the choice between having code inside the guest that
>>>>>>> spins, maybe even only checks every x ms, by programming a timer,
>>>>>>> or we can try to make an event out of the memory write. V1 was
>>>>>>> the former, v2 (this one) is the latter. This version performs a
>>>>>>> lot better and is easier to understand.
>>>>>> 
>>>>>> The abstraction layers should not be broken lightly, I suppose some
>>>>>> performance or laziness^Wlocal optimization reasons were behind vmport
>>>>>> design too. The ideal way to solve this could be to detect a spinning
>>>>>> CPU and optimize that for all architectures, that could be tricky
>>>>>> though (if a CPU remains in the same TB for extended periods, inspect
>>>>>> the TB: if it performs a loop with a single load instruction, replace
>>>>>> the load by a special wait operation for any memory stores to that
>>>>>> page).
>>>> 
>>>> How's that going to work with KVM?
>>>> 
>>>>> In fact, the whole kernel loading way we go today is pretty much
>>>>> wrong. We should rather do it similar to OpenBIOS where firmware
>>>>> always loads and then pulls the kernel from QEMU using a PV
>>>>> interface. At that point, we would have to implement such an
>>>>> optimization as you suggest. Or implement a hypercall :).
>>>> 
>>>> I think the current approach is more usable for most purposes.  If you
>>>> start U-Boot instead of a kernel, how do pass information on from the
>>>> user (kernel, rfs, etc)?  Require the user to create flash images[1]?
>>> 
>>> No, for example OpenBIOS gets the kernel command line from fw_cfg device.
>>> 
>>>> Maybe that's a useful mode of operation in some cases, but I don't think
>>>> we should be slavishly bound to it.  Think of the current approach as
>>>> something between whole-system and userspace emulation.
>>> 
>>> This is similar to ARM, M68k and Xtensa semi-hosting mode, but not at
>>> kernel level but lower. Perhaps this mode should be enabled with
>>> -semihosting flag or a new flag. Then the bare metal version could be
>>> run without the flag.
>> 
>> and then we'd have 2 implementations for running in system emulation mode 
>> and need to maintain both. I don't think that scales very well.
> 
> No, but such hacks are not common.
> 
>>> 
>>>> Where does the device tree come from?  How do you tell the guest about
>>>> what devices it has, especially in virtualization scenarios with non-PCI
>>>> passthrough devices, or custom qdev instantiations?
>>>> 
>>>>> But at least we'd always be running the same guest software stack.
>>>> 
>>>> No we wouldn't.  Any U-Boot that runs under QEMU would have to be
>>>> heavily modified, unless we want to implement a ton of random device
>>>> emulation, at least one extra memory translation layer (LAWs, localbus
>>>> windows, CCSRBAR, and such), hacks to allow locked cache lines to
>>>> operate despite a lack of backing store, etc.
>>> 
>>> I'd say HW emulation business as usual. Now with the new memory API,
>>> it should be possible to emulate the caches with line locking and TLBs
>>> etc., this was not previously possible. IIRC implementing locked cache
>>> lines would allow x86 to boot unmodified coreboot.
>> 
>> So how would you emulate cache lines with line locking on KVM?
> 
> The cache would be a MMIO device which registers to handle all memory
> space. Configuring the cache controller changes how the device
> operates. Put this device between CPU and memory and other devices.
> Performance would probably be horrible, so CPU should disable the
> device automatically after some time.

So how would you execute code on this region then? :)

> 
>> However, we already have a number of hacks in SeaBIOS to run in QEMU, so I 
>> don't see an issue in adding a few here and there in u-boot. The memory 
>> pressure is a real issue though. I'm not sure how we'd manage that one. 
>> Maybe we could try and reuse the host u-boot binary? heh
> 
> I don't think SeaBIOS breaks layering except for fw_cfg.

I'm not saying we're breaking layering there. I'm saying that changing u-boot 
is not so bad, since it's the same as we do with SeaBIOS. It was an argument in 
favor of your position.

> For extremely
> memory limited situation, perhaps QEMU (or Native KVM Tool for lean
> and mean version) could be run without glibc, inside kernel or even
> interfacing directly with the hypervisor. I'd also continue making it
> possible to disable building unused devices and features.

I'm pretty sure you're not the only one with that goal ;).


Alex