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Date:   Thu, 3 Nov 2022 15:10:07 -0700
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Subject: Re: [PATCH v6 13/21] gunyah: vm_mgr: Introduce basic VM Manager
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To:     Arnd Bergmann <arnd@arndb.de>,
        Bjorn Andersson <quic_bjorande@quicinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
CC:     Murali Nalajala <quic_mnalajal@quicinc.com>,
        Trilok Soni <quic_tsoni@quicinc.com>,
        Srivatsa Vaddagiri <quic_svaddagi@quicinc.com>,
        Carl van Schaik <quic_cvanscha@quicinc.com>,
        Prakruthi Deepak Heragu <quic_pheragu@quicinc.com>,
        Andy Gross <agross@kernel.org>,
        Jassi Brar <jassisinghbrar@gmail.com>,
        <linux-arm-kernel@lists.infradead.org>,
        "Mark Rutland" <mark.rutland@arm.com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
        Sudeep Holla <sudeep.holla@arm.com>,
        "Marc Zyngier" <maz@kernel.org>, Rob Herring <robh+dt@kernel.org>,
        "Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
        Jonathan Corbet <corbet@lwn.net>,
        Will Deacon <will@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Srinivas Kandagatla <srinivas.kandagatla@linaro.org>,
        Amol Maheshwari <amahesh@qti.qualcomm.com>,
        Kalle Valo <kvalo@kernel.org>, <devicetree@vger.kernel.org>,
        <linux-doc@vger.kernel.org>, <linux-arm-msm@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
References: <20221026185846.3983888-1-quic_eberman@quicinc.com>
 <20221026185846.3983888-14-quic_eberman@quicinc.com>
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 <7c59a115-36c5-c954-5610-ef5ef1dbb83e@quicinc.com>
 <a3754259-9989-495e-a6bd-5501daff06a2@app.fastmail.com>
From:   Elliot Berman <quic_eberman@quicinc.com>
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On 11/3/2022 2:39 AM, Arnd Bergmann wrote:
> On Wed, Nov 2, 2022, at 19:44, Elliot Berman wrote:
>> On 11/2/2022 12:31 AM, Arnd Bergmann wrote:
>>>> +static long gh_dev_ioctl_create_vm(unsigned long arg)
>>>> +{
>>>> +	struct gunyah_vm *ghvm;
>>>> +	struct file *file;
>>>> +	int fd, err;
>>>> +
>>>> +	/* arg reserved for future use. */
>>>> +	if (arg)
>>>> +		return -EINVAL;
>>>
>>> Do you have something specific in mind here? If 'create'
>>> is the only command you support, and it has no arguments,
>>> it would be easier to do it implicitly during open() and
>>> have each fd opened from /dev/gunyah represent a new VM.
>>>
>>
>> I'd like the argument here to support different types of virtual
>> machines. I want to leave open what "different types" can be in case
>> something new comes up in the future, but immediately "different type"
>> would correspond to a few different authentication mechanisms for
>> virtual machines that Gunyah supports.
>>
>> In this series, I'm only supporting unauthenticated virtual machines
>> because they are the simplest to get up and running from a Linux
>> userspace. When I introduce the other authentication mechanisms, I'll
>> expand much more on how they work, but I'll give quick overview here.
>> Other authentication mechanisms that are currently supported by Gunyah
>> are "protected VM" and, on Qualcomm platforms, "PIL/carveout VMs".
>> Protected VMs are *loosely* similar to the protected firmware design for
>> KVM and intended to support Android virtualization use cases.
>> PIL/carevout VMs are special virtual machines that can run on Qualcomm
>> firmware which authenticate in a way similar to remoteproc firmware (MDT
>> loader).
> 
> Ok, thanks for the background. Having different types of virtual
> machines does mean that you may need some complexity, but I would
> still lean towards using the simpler context management of opening
> the /dev/gunyah device node to get a new context, and then using
> ioctls on each fd to manage that context, instead of going through
> the extra indirection of having a secondary 'open context' command
> that always requires opening two file descriptors.
> 
>>> I'm correct, you can just turn the entire bus/device/driver
>>> structure within your code into simple function calls, where
>>> the main code calls vm_mgr_probe() as an exported function
>>> instead of creating a device.
>>
>> Ack. I can do this, although I am nervous about this snowballing into a
>> situation where I have a mega-module.
>>
>>   > Please stop beating everything in a single module.
>>
>> https://lore.kernel.org/all/250945d2-3940-9830-63e5-beec5f44010b@linaro.org/
> 
> I see you concern, but I wasn't suggesting having everything
> in one module either. There are three common ways of splitting
> things into separate modules:
> 
> - I suggested having the vm_mgr module as a library block that
>    exports a few symbols which get used by the core module. The
>    module doesn't do anything on its own, but loading the core
>    module forces loading the vm_mgr.
> 

Got the idea, I'll do this.

- Elliot

> - Alternatively one can do the opposite, and have symbols
>    exported by the core module, with the vm_mgr module using
>    it. This would make sense if you commonly have the core
>    module loaded on virtual machines that do not need to manage
>    other VMs.
> 
> - The method you have is to have a lower "bus" level that
>    abstracts device providers from consumers, with both sides
>    hooking into the bus. This makes sense for physical buses
>    like PCI or USB where both the host driver and the function
>    driver are unaware of implementation details of the other,
>    but in your case it does not seem like a good fit.
> 
>          Arnd