From: Chao Gao <chao.gao@intel.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: xen-devel@lists.xenproject.org
Subject: Re: [Xen-devel] Reset pass-thru devices in a VM
Date: Fri, 9 Aug 2019 21:24:14 +0800 [thread overview]
Message-ID: <20190809132412.GA12877@gao-cwp> (raw)
In-Reply-To: <e6e17b65-3d91-84fa-1831-571676503e2c@suse.com>
On Fri, Aug 09, 2019 at 10:49:32AM +0200, Jan Beulich wrote:
>On 09.08.2019 10:38, Chao Gao wrote:
>>I have a device which only supports secondary bus reset. After being
>>assigned to a VM, it would be placed under host bridge. For devices
>>under host bridge, secondary bus reset is not applicable. Thus, a VM
>>has no way to reset this device.
>>
>>This device's usage would be limited without PCI reset (for example, its
>>driver cannot re-initialize the device properly without PCI reset, which
>>means in VM device won't be usable after unloading the driver), it would
>>be much better if there is a way available to VMs to reset the device.
>>
>>In my mind, a straightfoward solution is to create a virtual bridge
>>for a VM and place the pass-thru device under a virtual bridge. But it
>>isn't supported in Xen (KVM/QEMU supports) and enabling it looks need
>>a lot of efforts.
>
>Meanwhile I think a couple of years ago there was some initial effort
>to get a newer chipset (Q35 iirc) emulated for HVM guests.
Yes. But it seems that no one is working on this feature now.
>
>>Alternatively, emulating FLR (Function Level Reset)
>>capability for this device might be a feasible way and only needs
>>relatively few changes. I am planning to enable an opt-in feature
>>(like 'permissive') to allow qemu to expose FLR capability to guest for
>>pass-thru devices as long as this device is resetable on dom0 (i.e. the
>>device has 'reset' attribute under its sysfs). And when guest initiates
>>an FLR, qemu just echo 1 to the 'reset' attribute on dom0.
>>
>>Do you think emulating FLR capability is doable?
>
>Wouldn't a such emulated guest initiated reset affect other devices
>(likely not under control of this guest) as well?
No. Linux kernel guarantees that reset to a device won't affect
other devices. Otherwise, such device cannot be reset and no
'reset' attribute will be created under device's sysfs.
Specfically, the invocation of pci_dev_reset_slot_function() and
pci_parent_bus_reset() in pci_probe_reset_function() will check whether
the device (function) is the only one under the slot or bus
respectively. In pci_create_capabilities_sysfs(), 'reset' attribute is
created only if dev->reset_fn is not zero.
Thanks
Chao
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next prev parent reply other threads:[~2019-08-09 13:20 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-09 8:38 [Xen-devel] Reset pass-thru devices in a VM Chao Gao
2019-08-09 8:49 ` Jan Beulich
2019-08-09 13:24 ` Chao Gao [this message]
2019-08-09 13:23 ` Jan Beulich
2019-08-09 14:13 ` Chao Gao
2019-08-09 12:42 ` Roger Pau Monné
2019-08-09 13:57 ` Chao Gao
2019-08-26 21:17 ` Pasi Kärkkäinen
2019-08-27 4:40 ` Chao Gao
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