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From: Julien Grall <julien@xen.org>
To: Jan Beulich <jbeulich@suse.com>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Ian Jackson <iwj@xenproject.org>, Wei Liu <wl@xen.org>,
	Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: Ping: [PATCH v5 0/6] evtchn: (not so) recent XSAs follow-on
Date: Wed, 21 Apr 2021 16:56:13 +0100	[thread overview]
Message-ID: <40e90456-90dc-7932-68ec-6f4d0941999f@xen.org> (raw)
In-Reply-To: <d29fa89b-ea0a-bdbd-04c9-02eff0854d47@suse.com>



On 21/04/2021 16:23, Jan Beulich wrote:
> On 27.01.2021 09:13, Jan Beulich wrote:
>> These are grouped into a series largely because of their origin,
>> not so much because there are (heavy) dependencies among them.
>> The main change from v4 is the dropping of the two patches trying
>> to do away with the double event lock acquires in interdomain
>> channel handling. See also the individual patches.
>>
>> 1: use per-channel lock where possible
>> 2: convert domain event lock to an r/w one
>> 3: slightly defer lock acquire where possible
>> 4: add helper for port_is_valid() + evtchn_from_port()
>> 5: type adjustments
>> 6: drop acquiring of per-channel lock from send_guest_{global,vcpu}_virq()
> 
> Only patch 4 here has got an ack so far - may I ask for clear feedback
> as to at least some of these being acceptable (I can see the last one
> being controversial, and if this was the only one left I probably
> wouldn't even ping, despite thinking that it helps reduce unecessary
> overhead).

I left feedback for the series one the previous version (see [1]). It 
would have been nice is if it was mentionned somewhere as this is still 
unresolved.

The locking in the event channel is already quite fragile (see recent 
XSAs, follow-up bugs...). Even if the pattern is used somewhere (as you 
suggested), I don't think it is good idea to pertain it.

To be clear, I am not saying I don't care about performance. Instead I 
am trying to find a trade off between code maintenability and 
performance. So far, I didn't see any data justifying that the extra 
performance is worth the risk of making a code more fragile.

Cheers,

[1] <3c393170-09f9-6d31-c227-b599f8769e35@xen.org>

> 
> Jan
> 

-- 
Julien Grall


  reply	other threads:[~2021-04-21 15:56 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-27  8:13 [PATCH v5 0/6] evtchn: (not so) recent XSAs follow-on Jan Beulich
2021-01-27  8:15 ` [PATCH v5 1/6] evtchn: use per-channel lock where possible Jan Beulich
2021-01-27  8:16 ` [PATCH v5 2/6] evtchn: convert domain event lock to an r/w one Jan Beulich
2021-05-27 11:01   ` Roger Pau Monné
2021-05-27 11:16     ` Jan Beulich
2022-07-07 18:00   ` Julien Grall
2021-01-27  8:16 ` [PATCH v5 3/6] evtchn: slightly defer lock acquire where possible Jan Beulich
2021-01-27  8:16 ` [PATCH v5 4/6] evtchn: add helper for port_is_valid() + evtchn_from_port() Jan Beulich
2021-01-27  8:17 ` [PATCH v5 5/6] evtchn: type adjustments Jan Beulich
2021-01-27  8:17 ` [PATCH v5 6/6] evtchn: drop acquiring of per-channel lock from send_guest_{global,vcpu}_virq() Jan Beulich
2021-04-21 15:23 ` Ping: [PATCH v5 0/6] evtchn: (not so) recent XSAs follow-on Jan Beulich
2021-04-21 15:56   ` Julien Grall [this message]
2021-04-22  8:53     ` Jan Beulich
2021-05-14 15:29       ` Roger Pau Monné
2021-05-17  7:15         ` Jan Beulich

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