linux-kernel - Re: [PATCH v5 13/13] tty: gunyah: Add tty console driver for RM Console Services

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Message-ID: <0640f0a5-19c1-55d2-229a-37751a18118f@quicinc.com>
Date:   Thu, 13 Oct 2022 13:54:36 -0700
From:   Elliot Berman <quic_eberman@...cinc.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
CC:     Arnd Bergmann <arnd@...db.de>, Jiri Slaby <jirislaby@...nel.org>,
        "Bjorn Andersson" <quic_bjorande@...cinc.com>,
        Murali Nalajala <quic_mnalajal@...cinc.com>,
        Trilok Soni <quic_tsoni@...cinc.com>,
        "Srivatsa Vaddagiri" <quic_svaddagi@...cinc.com>,
        Carl van Schaik <quic_cvanscha@...cinc.com>,
        Prakruthi Deepak Heragu <quic_pheragu@...cinc.com>,
        Andy Gross <agross@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Jassi Brar <jassisinghbrar@...il.com>,
        <linux-arm-kernel@...ts.infradead.org>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Sudeep Holla <sudeep.holla@....com>,
        Marc Zyngier <maz@...nel.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        "Will Deacon" <will@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        <devicetree@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-arm-msm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v5 13/13] tty: gunyah: Add tty console driver for RM
 Console Services



On 10/11/2022 11:55 PM, Greg Kroah-Hartman wrote:
> On Tue, Oct 11, 2022 at 03:04:47PM -0700, Elliot Berman wrote:
>>
>>
>> On 10/11/2022 4:09 AM, Arnd Bergmann wrote:
>>> On Tue, Oct 11, 2022, at 8:02 AM, Jiri Slaby wrote:
>>>> On 11. 10. 22, 2:08, Elliot Berman wrote:
>>>>> +
>>>>> +	/* below are for printk console.
>>>>> +	 * gh_rm_console_* calls will sleep and console_write can be called from
>>>>> +	 * atomic ctx. Two xmit buffers are used. The active buffer is tracked with
>>>>> +	 * co_xmit_idx. Writes go into the co_xmit_buf[co_xmit_idx] buffer.
>>>>> +	 * A work is scheduled to flush the bytes. The work will swap the active buffer
>>>>> +	 * and write out the other buffer.
>>>>> +	 */
>>>>
>>>> Ugh, why? This is too ugly and unnecessary. What about passing the kfifo
>>>> to gh_rm_console_write() instead? You do memcpy() there anyway.
>>>
>>> Another problem here is that you really want the console output to be
>>> printed from atomic context, otherwise one would never see e.g. the
>>> output of a panic() call. Having a deferred write is probably fine for
>>> normal tty operations, but you probably want a different device for the
>>> console here, e.g. the hvc_dcc driver.
>>>
>>
>> Yes, that is our perspective on the RM console driver as well. I'll make
>> this more explicit in the Kconfig/commit text. We expect most VMs
>> (especially Linux) to use some other console mechanism provided by their
>> VMM. I'm submitting here because we are presently using RM console on some
>> of our VMs where we have other ways to collects logs on panic. It also makes
>> it easier to implement a simple virtual machine manager that does not want
>> to virtualize a serial device or have a virtio stack.
> 
> The whole goal of virtio was so that we would not have all of these
> random custom drivers for new hypervisors all over the place, requiring
> custom userspace interaction with them.
> 
> Please use virtio, that's what it is there for, don't create a new
> console device if you do not have to.

We have a lightweight VM product use case today that doesn't want to 
support an entire virtio stack just for a console. This VM already has a 
Gunyah stack present, and to facilitate their console needs, we want to 
give them the Gunyah console.

There are a few other hypervisors that also provide a console facility 
in Linux: Xen, ePAPR hypervisor and z/VM.

Thanks,
Elliot