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Re: [Xen-devel] [PATCH] xen: Avoid calling device suspend/resume callbacks
- To: Jan Beulich <JBeulich@xxxxxxxx>, Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 29 Jul 2019 18:06:09 +0100
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- Cc: Juergen Gross <JGross@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>
- Delivery-date: Mon, 29 Jul 2019 17:06:45 +0000
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On 29/07/2019 16:57, Jan Beulich wrote:
> On 29.07.2019 17:41, Ross Lagerwall wrote:
>> When suspending/resuming or migrating under Xen, there isn't much need
>> for suspending and resuming all the attached devices since the Xen/QEMU
>> should correctly maintain the hardware state. Drop these calls and
>> replace with more specific calls to ensure Xen frontend devices are
>> properly reconnected.
> Is this true for the general pass-through case as well? While migration
> may not be (fully) compatible with pass-through, iirc save/restore is.
What gives you this impression?
Migration and save/restore are *literally* the same thing, except that
in one case you're piping the data to/from disk, and in the other you're
piping it to the destination and restoring it immediately.
If you look at the toolstack code, it is all in terms of reading/writing
an fd (including libxl's API) which is either a network socket or a
file, as chosen by xl.
> Would qemu restore state of physical PCI devices?
What state would Qemu be in a position to know about, which isn't
already present in Qemu's datablob?
What we do with graphics cards is to merge Xens logdirty bitmap, with a
dirty list provided by the card itself. This needs a device-specific
knowledge. In addition, there is an opaque blob of data produced by the
source card, which is handed to the destination card. That also lives
in the stream.
Intel's Scalable IOV spec is attempting to rationalise this by having a
standard ways of getting logdirty and "internal state" information out
of a device, but for the moment, it requires custom device-driver
specific code to do anything migration related with real hardware.
As for why its safe to do like this, the best argument is that this is
how all other vendors do migration, including KVM. Xen is the
odd-one-out using the full S3 path.
~Andrew
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