Re: [Qemu-devel] [PATCH] qdev: Reset hotplugged devices

[Isaku Yamahata]

I added CC for those who might be interested in this discussion.

On Thu, Aug 26, 2010 at 08:02:38AM -0500, Anthony Liguori wrote:
> On 08/26/2010 03:38 AM, Isaku Yamahata wrote:
>>
>>> I think that starts by understanding exactly what's guaranteed and
>>> understanding what the use cases are for it.
>>>      
>> Fair enough. How about the followings?
>>    
>
> Thanks for enumerating.
>
>> This is just a starting point. I borrowed terminology pci/pcie spec.
>>
>>
>> reset
>>      Bring the state of hardware state to consistent state.
>>      (some state might be left unknown.)
>>      
>>
>> system reset
>>      a hardware mechanism for setting or returning all hardware states
>>      to the initial conditions.
>>      Use case:
>>      In qemu, system_system_reset().
>>
>>
>> cold reset(power on reset)
>>      system reset following the application of power.
>>      Use case:
>>      In qemu, system_reset() in main().
>>      We might want to use this as a power cycle.
>>      When a device is hot plugged, the device should be cold reset too.
>>      This is your motivation.
>>      QEMU_RESET_COLD
>>      Guarantee:
>>      The internal status must be same to qdev_init() + qdev_reset()
>>    
>
> This is what we do today in QEMU and from a functional perspective it  
> covers the type of function we need today.
>
>>      
>> warm reset
>>      system reset without cycling the supplied power.
>>      Use case:
>>      In qemu, system_reset() in main_loop(). There are many places
>>      which calls qemu_system_reset_request().
>>      Some state are retained across warm reset. Like PCIe AER, error
>>      reporting registers need to keep its contents across warm reset
>>      as OS would examine them and report it when hardware errors caused
>>      warm reset.
>>      QEMU_RESET_WARM
>>    
>
> With AER, I can't imagine that this matters that much unless we're doing  
> PCI passthrough, right?

Even without PCI passthrough, AER errors can be injected into emulated pci/pcie
devices in a virtual pcie bus hierarchy from qemu command line.
This is useful to test guest OS AER handler.


> So maybe the way we should frame this discussion is, what's the type of  
> reset semantics that we need to support for PCI passthrough?  The next  
> question after that is how do we achieve the different types of reset  
> for passthrough devices?

What I want is hot reset in pcie terminology on a virtual bus as
PCI_BRIDGE_CTL_BUS_RESET emulation and propagated reset on devices/child
buses which might be a directly assigned.
In direct assigned device case, device-assignment code would be notified the 
reset.

As hot reset has same effect to warm reset in functionality sense
(the difference is the way to signal it in physical/signal layer which
qemu doesn't care),
I'd like to implement pci bus reset as triggering warm reset on a
(virtual) bus by utilizing qdev frame work.
This would be applicable to ata, scsi, I suppose.


It's another story how to virtualize hot reset on a given directly assigned
pci function or a pcie bus hierarchy. For example, as PCI device assignment
is done per function basis, co-existing functions in the same card shouldn't
be reset.
Another example is, virtual pci bus hierarchy might be reset, but it would
be difficult problem how to map the virtual bus topology to host bus topology.

thanks,

> BTW, if you could transfer some of this discussion to a wiki page on  
> qemu.org, I think that would be extremely valuable.
>
> Regards,
>
> Anthony Liguori
>
>> bus reset
>>      Reset bus and devices on the bus.
>>      Bus reset is usually triggered when cold reset, warm reset and
>>      commanding the bus controller to reset the child bus.
>>      When bus reset is triggered as command to bus controller,
>>      the effect is usually same to warm reset on devices on the bus.
>>
>>      Typically on parallel bus, bus reset is started by asserting
>>      a designated signal.
>>      Example: PCI RST#, ATA RESET-, SCSI RST
>>      
>>      Use case:
>>      bus reset as result of programming bus controller.
>>      Qemu is currently missing it which I'd like to fill for pci bus.
>>      ATA and SCSI could benefit from this.
>>      QEMU_RESET_WARM with bus.
>>      Guarantee:
>>      device state under the bus is same as warm reset.
>>
>>
>> device/function reset:
>>      Reset triggered by sending reset command to a device.
>>      This is bus/device specific.
>>      There might be many reset commands whose effects are different.
>>      Example: PCI FLR, ATA DEVICE RESET command,
>>                   scsi bus device reset message.
>>
>>      This reset is bus specific, so it wouldn't be suitable for qdev
>>      frame work and could be handled by each bus level.
>>
>>      
>> hot reset:
>>      I just put it here for completeness because pcie defines hot reset.
>>      A reset propagated in-band across a Link using a Physical Layer
>>      mechanism.
>>      Qemu doesn't emulate physical layer, so we don't care it.
>>      From software point of view, hot reset has same effect to warm reset.
>>
>>    
>

-- 
yamahata