Re: [PATCH 2/2] x86/hvm/ioreq: allow ioreq servers to use HVM_PARAM_[BUF]IOREQ_PFN

["Jan Beulich" <JBeulich@suse.com>]

>>> On 08.10.18 at 16:38, <Paul.Durrant@citrix.com> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@suse.com]
>> Sent: 08 October 2018 14:29
>> To: Paul Durrant <Paul.Durrant@citrix.com>
>> Cc: Andrew Cooper <Andrew.Cooper3@citrix.com>; Wei Liu
>> <wei.liu2@citrix.com>; xen-devel <xen-devel@lists.xenproject.org>
>> Subject: Re: [PATCH 2/2] x86/hvm/ioreq: allow ioreq servers to use
>> HVM_PARAM_[BUF]IOREQ_PFN
>> 
>> >>> On 05.10.18 at 15:43, <paul.durrant@citrix.com> wrote:
>> > Since commit 2c257bd6 "x86/hvm: remove default ioreq server (again)" the
>> > GFNs allocated by the toolstack and set in HVM_PARAM_IOREQ_PFN and
>> > HVM_PARAM_BUFIOREQ_PFN have been unused. This patch allows them to be
>> used
>> > by (non-default) ioreq servers.
>> >
>> > NOTE: This fixes a compatibility issue. A guest created on a version of
>> >       Xen that pre-dates the initial ioreq server implementation and
>> then
>> >       migrated in will currently fail to resume because its migration
>> >       stream will lack values for HVM_PARAM_IOREQ_SERVER_PFN and
>> >       HVM_PARAM_NR_IOREQ_SERVER_PAGES *unless* the system has an
>> >       emulator domain that uses direct resource mapping (which depends
>> >       on the version of privcmd it happens to have) in which case it
>> >       will not require use of GFNs for the ioreq server shared
>> >       pages.
>> 
>> Meaning this wants to be backported till where?
>> 
> 
> This fix is 4.12 specific because it is predicated on removal of default 
> ioreq server support.

Ah, good.

>> > A similar compatibility issue with migrated-in VMs exists with Xen 4.11
>> > because the upstream QEMU fall-back to use legacy ioreq server was
>> broken
>> > when direct resource mapping was introduced.
>> > This is because, prior to the resource mapping patches, it was the
>> creation
>> > of the non-default ioreq server that failed if GFNs were not available
>> > whereas, as of 4.11, it is retrieval of the info that fails which does
>> not
>> > trigger the fall-back.
>> 
>> Are you working on a fix or workaround for this, too, then?
>> 
> 
> Not yet. I'm not sure how to approach this. There are a few options:
> 
> 1. Backport default IOREQ server removal and this fix
> 2. Do a bespoke 4.11 fix that forces IOREQ server creation to fail if there 
> are no GFNs available, thus triggering the default IOREQ server fallback in 
> QEMU.
> 3. Upstream a fix into QEMU to do a fallback at the point that it fails to 
> get GFNs i.e. have it close its IOREQ server and then fall back to default.
> 
> The problem with 1 is that it breaks qemu trad. 2 is probably simplest, but 
> if the emualator can do resource mapping it is unnecessary. 3 is probably 
> best, but it's not our fix to deliver.
> 
> Thoughts?

2 indeed looks best to me then. Though I'm not sure I understand
what you say correctly: Would triggering the default IOREQ server
fallback be a step backwards, if the emulator is capable and able to
use resource mapping? If so, somehow avoiding this would of
course be nice, and I'd then assume this isn't reasonable to achieve
without a qemu side change, in which case the solution wouldn't be
any better than 3 anymore.

Jan



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