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

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] qdev: Reset hotplugged devices
Date: Thu, 26 Aug 2010 08:04:44 -0500
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On 08/26/2010 03:38 AM, Isaku Yamahata wrote:


BTW, just from a implementation perspective, I'd rather have multiple reset callbacks in qdev instead of having a single callback with a type flag. A type flag implies that every callback has to handle all cases whereas with separate callbacks, if a device doesn't implement warm_reset we can easily default it to reset (which is a cold reset).


Anthony Liguori

        The internal status must be same to qdev_init() + qdev_reset()

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.

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.
        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
        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.

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