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Re: [Qemu-block] [Qemu-devel] [PATCH] ahci: enable pci bus master Memory

From: John Snow
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] ahci: enable pci bus master MemoryRegion before loading ahci engines
Date: Tue, 10 Sep 2019 10:08:20 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 9/10/19 9:58 AM, Michael S. Tsirkin wrote:
> On Tue, Sep 10, 2019 at 09:50:41AM -0400, John Snow wrote:
>> On 9/10/19 3:04 AM, Michael S. Tsirkin wrote:
>>> On Tue, Sep 10, 2019 at 01:18:37AM +0800, andychiu wrote:
>>>> If Windows 10 guests have enabled 'turn off hard disk after idle'
>>>> option in power settings, and the guest has a SATA disk plugged in,
>>>> the SATA disk will be turned off after a specified idle time.
>>>> If the guest is live migrated or saved/loaded with its SATA disk
>>>> turned off, the following error will occur:
>>>> qemu-system-x86_64: AHCI: Failed to start FIS receive engine: bad FIS 
>>>> receive buffer address
>>>> qemu-system-x86_64: Failed to load ich9_ahci:ahci
>>>> qemu-system-x86_64: error while loading state for instance 0x0 of device 
>>>> '0000:00:1a.0/ich9_ahci'
>>>> qemu-system-x86_64: load of migration failed: Operation not permitted
>>>> Observation from trace logs shows that a while after Windows 10 turns off
>>>> a SATA disk (IDE disks don't have the following behavior),
>>>> it will disable the PCI_COMMAND_MASTER flag of the pci device containing
>>>> the ahci device. When the the disk is turning back on,
>>>> the PCI_COMMAND_MASTER flag will be restored first.
>>>> But if the guest is migrated or saved/loaded while the disk is off,
>>>> the post_load callback of ahci device, ahci_state_post_load(), will fail
>>>> at ahci_cond_start_engines() if the MemoryRegion
>>>> pci_dev->bus_master_enable_region is not enabled, with pci_dev pointing
>>>> to the PCIDevice struct containing the ahci device.
>>>> This patch enables pci_dev->bus_master_enable_region before calling
>>>> ahci_cond_start_engines() in ahci_state_post_load(), and restore the
>>>> MemoryRegion to its original state afterwards.
>>>> Signed-off-by: andychiu <address@hidden>
>>> Poking at PCI device internals like this seems fragile.  And force
>>> enabling bus master can lead to unpleasantness like corrupting guest
>>> memory, unhandled interrupts, etc.  E.g. it's quite reasonable,
>>> spec-wise, for the guest to move thing in memory around while bus
>>> mastering is off.
>>> Can you teach ahci that region being disabled
>>> during migration is ok, and recover from it?
>> That's what I'm wondering.
>> I could try to just disable the FIS RX engine if the mapping fails, but
>> that will require a change to guest visible state.
>> My hunch, though, is that when windows re-enables the device it will
>> need to re-program the address registers anyway, so it might cope well
>> with the FIS RX bit getting switched off.
>> (I'm wondering if it isn't a mistake that QEMU is trying to re-map this
>> address in the first place. Is it legal that the PCI device has pci bus
>> master disabled but we've held on to a mapping?
> If you are poking at guest memory when bus master is off, then most likely 
> yes.
>> Should there be some
>> callback where AHCI knows to invalidate mappings at that point...?)
> ATM the callback is the config write, you check
> proxy->pci_dev.config[PCI_COMMAND] & PCI_COMMAND_MASTER
> and if disabled invalidate the mapping.
> virtio at least has code that pokes at
> proxy->pci_dev.config[PCI_COMMAND] too, I'm quite
> open to a function along the lines of
> pci_is_bus_master_enabled()
> that will do this.

Well, that's not a callback. I don't think it's right to check the
PCI_COMMAND register *every* time AHCI does anything at all to see if
its mappings are still valid.

AHCI makes a mapping *once* when FIS RX is turned on, and it unmaps it
when it's turned off. It assumes it remains valid that whole time. When
we migrate, it checks to see if it was running, and performs the
mappings again to re-boot the state machine.

What I'm asking is; what are the implications of a guest disabling
PCI_COMMAND_MASTER? (I don't know PCI as well as you do.)

What should that mean for the AHCI state machine?

Does this *necessarily* invalidate the mappings?
(In which case -- it's an error that AHCI held on to them after Windows
disabled the card, even if AHCI isn't being engaged by the guest
anymore. Essentially, we were turned off but didn't clean up a dangling
pointer, but we need the event that tells us to clean the dangling mapping.)

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