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Message-ID: <13adfc8d-8118-2fd7-3a66-98dfbf8037a9@linux.intel.com>
Date:   Fri, 21 Oct 2022 16:51:34 -0700
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Subject: Re: [PATCH v15 2/3] virt: Add TDX guest driver
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To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        Shuah Khan <shuah@kernel.org>,
        Jonathan Corbet <corbet@lwn.net>,
        "H . Peter Anvin" <hpa@zytor.com>,
        "Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Kai Huang <kai.huang@intel.com>,
        Wander Lairson Costa <wander@redhat.com>,
        Isaku Yamahata <isaku.yamahata@gmail.com>,
        marcelo.cerri@canonical.com, tim.gardner@canonical.com,
        khalid.elmously@canonical.com, philip.cox@canonical.com,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        linux-doc@vger.kernel.org
References: <20221020045828.2354731-1-sathyanarayanan.kuppuswamy@linux.intel.com>
 <20221020045828.2354731-3-sathyanarayanan.kuppuswamy@linux.intel.com>
 <Y1De4IyAB6n2qs4V@kroah.com>
 <34ef18d6-69f8-853a-d1ba-7023822e17ff@linux.intel.com>
 <Y1Iimg0WItgIGq6/@kroah.com>
From:   Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
In-Reply-To: <Y1Iimg0WItgIGq6/@kroah.com>
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Precedence: bulk

Hi Greg,

On 10/20/22 9:39 PM, Greg Kroah-Hartman wrote:
>>>> +#ifdef MODULE
>>>> +static const struct x86_cpu_id tdx_guest_ids[] = {
>>>> +	X86_MATCH_FEATURE(X86_FEATURE_TDX_GUEST, NULL),
>>>> +	{}
>>>> +};
>>>> +MODULE_DEVICE_TABLE(x86cpu, tdx_guest_ids);
>>>> +#endif
>>> Why the #ifdef?  Should not be needed, right?
>> I have added it to fix the following warning reported by 0-day.
>>
>> https://lore.kernel.org/lkml/202209211607.tCtTWKbV-lkp@intel.com/
>>
>> It is related to nullifying the MODULE_DEVICE_TABLE in #ifndef MODULE
>> case in linux/module.h.
> Then fix it properly, by correctly using that structure no matter what.
> You don't do that here...

I think we can use __maybe_unused attribute to fix this warning like
mentioned below. Are you fine with it?

--- a/drivers/virt/coco/tdx-guest/tdx-guest.c
+++ b/drivers/virt/coco/tdx-guest/tdx-guest.c
@@ -118,13 +118,11 @@ static void __exit tdx_guest_exit(void)
 }
 module_exit(tdx_guest_exit);
 
-#ifdef MODULE
-static const struct x86_cpu_id tdx_guest_ids[] = {
+static const struct x86_cpu_id __maybe_unused tdx_guest_ids[] = {
        X86_MATCH_FEATURE(X86_FEATURE_TDX_GUEST, NULL),
        {}
 };
 MODULE_DEVICE_TABLE(x86cpu, tdx_guest_ids);
-#endif

Solution 2:
-----------

We can also modify the code to use this structure in all cases like
below. But it requires me to use slower x86_match_cpu() in place of 
cpu_feature_enabled() which I think is unnecessary.

--- a/drivers/virt/coco/tdx-guest/tdx-guest.c
+++ b/drivers/virt/coco/tdx-guest/tdx-guest.c
@@ -103,9 +103,15 @@ static struct miscdevice tdx_misc_dev = {
        .fops = &tdx_guest_fops,
 };
 
+static const struct x86_cpu_id tdx_guest_ids[] = {
+       X86_MATCH_FEATURE(X86_FEATURE_TDX_GUEST, NULL),
+       {}
+};
+MODULE_DEVICE_TABLE(x86cpu, tdx_guest_ids);
+
 static int __init tdx_guest_init(void)
 {
-       if (!cpu_feature_enabled(X86_FEATURE_TDX_GUEST))
+       if (!x86_match_cpu(tdx_guest_ids))
                return -ENODEV;
 
        return misc_register(&tdx_misc_dev);
@@ -118,14 +124,6 @@ static void __exit tdx_guest_exit(void)
 }
 module_exit(tdx_guest_exit);
 
-#ifdef MODULE
-static const struct x86_cpu_id tdx_guest_ids[] = {
-       X86_MATCH_FEATURE(X86_FEATURE_TDX_GUEST, NULL),
-       {}
-};
-MODULE_DEVICE_TABLE(x86cpu, tdx_guest_ids);
-#endif

-- 
Sathyanarayanan Kuppuswamy
Linux Kernel Developer