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Re: [Xen-devel] [PATCH RFC 0/6] Slotted channels for sync vm_events
- To: Petre Ovidiu PIRCALABU <ppircalabu@xxxxxxxxxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- From: George Dunlap <george.dunlap@xxxxxxxxxx>
- Date: Thu, 7 Feb 2019 11:46:56 +0000
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Ian Jackson <ian.jackson@xxxxxxxxxxxxx>, Tim Deegan <tim@xxxxxxx>, Julien Grall <julien.grall@xxxxxxx>, Tamas K Lengyel <tamas@xxxxxxxxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Delivery-date: Thu, 07 Feb 2019 11:47:15 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 2/6/19 2:26 PM, Petre Ovidiu PIRCALABU wrote:
> On Wed, 2018-12-19 at 20:52 +0200, Petre Pircalabu wrote:
>> This patchset is a rework of the "multi-page ring buffer" for
>> vm_events
>> patch based on Andrew Cooper's comments.
>> For synchronous vm_events the ring waitqueue logic was unnecessary as
>> the
>> vcpu sending the request was blocked until a response was received.
>> To simplify the request/response mechanism, an array of slotted
>> channels
>> was created, one per vcpu. Each vcpu puts the request in the
>> corresponding slot and blocks until the response is received.
>>
>> I'm sending this patch as a RFC because, while I'm still working on
>> way to
>> measure the overall performance improvement, your feedback would be a
>> great
>> assistance.
>>
>
> Is anyone still using asynchronous vm_event requests? (the vcpu is not
> blocked and no response is expected).
> If not, I suggest that the feature should be removed as it
> (significantly) increases the complexity of the current implementation.
Could you describe in a bit more detail what the situation is? What's
the current state fo affairs with vm_events, what you're trying to
change, why async vm_events is more difficult?
I certainly think it would be better if you could write the new vm_event
interface without having to spend a lot of effort supporting modes that
you think nobody uses.
On the other hand, getting into the habit of breaking stuff, even for
people we don't know about, will be a hindrance to community growth; a
commitment to keeping it working will be a benefit to growth.
But of course, we haven't declared the vm_event interface 'supported'
(it's not even mentioned in the SUPPORT.md document yet).
Just for the sake of discussion, would it be possible / reasonble, for
instance, to create a new interface, vm_events2, instead? Then you
could write it to share the ioreq interface without having legacy
baggage you're not using; we could deprecate and eventually remove
vm_events1, and if anyone shouts, we can put it back.
Thoughts?
-George
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