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

From: Blue Swirl
Subject: Re: [Qemu-ppc] [PATCH 24/58] PPC: E500: Add PV spinning code
Date: Tue, 27 Sep 2011 17:20:08 +0000

On Tue, Sep 27, 2011 at 5:03 PM, Alexander Graf <address@hidden> wrote:
> 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? :)

Easy, fix QEMU to allow executing from MMIO. (Yeah, I forgot about that).

>>> 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.

Never mind then ;-)

>> 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 ;).

Great, let's do it.

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