linux-kernel - Re: [PATCH v8 05/28] virt: gunyah: Add hypercalls to identify Gunyah

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Message-ID: <59bddf7a-f420-cccd-8029-65052f834c62@quicinc.com>
Date:   Tue, 17 Jan 2023 14:29:31 -0800
From:   Elliot Berman <quic_eberman@...cinc.com>
To:     Alex Elder <elder@...aro.org>,
        Bjorn Andersson <quic_bjorande@...cinc.com>,
        Catalin Marinas <catalin.marinas@....com>,
        "Will Deacon" <will@...nel.org>,
        Murali Nalajala <quic_mnalajal@...cinc.com>
CC:     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>,
        Arnd Bergmann <arnd@...db.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Jonathan Corbet <corbet@....net>,
        "Bagas Sanjaya" <bagasdotme@...il.com>,
        Jassi Brar <jassisinghbrar@...il.com>,
        "Sudeep Holla" <sudeep.holla@....com>,
        Mark Rutland <mark.rutland@....com>,
        "Lorenzo Pieralisi" <lpieralisi@...nel.org>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        <linux-arm-msm@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH v8 05/28] virt: gunyah: Add hypercalls to identify Gunyah



On 1/17/2023 11:20 AM, Alex Elder wrote:
> On 1/10/23 11:56 AM, Elliot Berman wrote:
>>>> There are two calls to help identify Gunyah:
>>>>
>>>> 1. gh_hypercall_get_uid() returns a UID when running under a Gunyah
>>>>     hypervisor.
>>>> 2. gh_hypercall_hyp_identify() returns build information and a set of
>>>>     feature flags that are supported by Gunyah.
>>>
>>> The first is a "service", while the second is a "hypercall".
>>> Can you explain the distinction?  The sentence at the top
>>> refers to both as "hypercalls".
>>>
>>
>> I learned more details about this to answer your question. "get_uid()" 
>> is a standardized call that is ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID 
>> defined in include/arm-smccc.h. I'll use that.
> 
> You didn't really explain the distinction between hypercall
> and service in Gunyah.  Both are encoded as "vendor specific
> hypervisor service calls" according to the SVCCC specification.
> I haven't found where ARM_SMCCC_VENDOR_HYP_CALL_UID_FUNC_ID
> gets handled, but I accept your answer that it's basically
> a standard call.  The "get UID" is the only one that's defined
> as a Gunyah "service"; the rest are hypercalls.
> 

This came from a Gunyah implementation detail that separates standard 
(service) calls from the non-standard calls. I was following the 
distinction from Gunyah code not realizing that it's actually a 
standardized call that Linux would already have support for.

Thanks,
Elliot

> It's not a big deal, I just noticed the difference and was
> curious about it.
> 
>                      -Alex