Re: [PATCH v15 2/3] virt: Add TDX guest driver

[Sathyanarayanan Kuppuswamy <sathyanarayanan.kuppuswamy@xxxxxxxxxxxxxxx>]

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@xxxxxxxxx/
>>
>> 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