linux-kernel - Re: [PATCH v7 09/18] virt: acrn: Introduce I/O request management

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Message-ID: <20210112060527.GF22447@shuo-intel.sh.intel.com>
Date:   Tue, 12 Jan 2021 14:05:27 +0800
From:   Shuo A Liu <shuo.a.liu@...el.com>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     <linux-kernel@...r.kernel.org>, <x86@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "Borislav Petkov" <bp@...en8.de>, Yu Wang <yu1.wang@...el.com>,
        Reinette Chatre <reinette.chatre@...el.com>,
        Zhi Wang <zhi.a.wang@...el.com>,
        Zhenyu Wang <zhenyuw@...ux.intel.com>
Subject: Re: [PATCH v7 09/18] virt: acrn: Introduce I/O request management

On Mon 11.Jan'21 at 13:52:19 -0800, Davidlohr Bueso wrote:
>On Wed, 06 Jan 2021, shuo.a.liu@...el.com wrote:
>>The processing flow of I/O requests are listed as following:
>>
>>a) The I/O handler of the hypervisor will fill an I/O request with
>>  PENDING state when a trapped I/O access happens in a User VM.
>>b) The hypervisor makes an upcall, which is a notification interrupt, to
>>  the Service VM.
>>c) The upcall handler schedules a tasklet to dispatch I/O requests.
>>d) The tasklet looks for the PENDING I/O requests, assigns them to
>>  different registered clients based on the address of the I/O accesses,
>>  updates their state to PROCESSING, and notifies the corresponding
>>  client to handle.
>
>Hmm so tasklets are deprecated (and have been for a while) and it's sad
>to see incoming new users in modern Linux. This wouldn't be the first one,
>however. We should be _removing_ users, not adding... In addition, this
>expands the whole tasklet_disable/enable() hacks.

Sorry, i had not noticed that.

>
>Could this not be done in process context instead?

It could be. The original consideration with tasklet was more about
performance as the I/O requests dispatching is a hot code path. I think
irq thread has little performance impact? I can have a try to convert
the tasklet to irq thread.

Thanks
shuo