linux-kernel - Re: [PATCH v5 1/5] hyperv: Define struct hv_output_get_vp

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Message-ID: <e43633f6-c303-439d-8dbe-730a5762753c@linux.microsoft.com>
Date: Mon, 6 Jan 2025 12:24:32 -0800
From: Roman Kisel <romank@...ux.microsoft.com>
To: Michael Kelley <mhklinux@...look.com>, "hpa@...or.com" <hpa@...or.com>,
 "kys@...rosoft.com" <kys@...rosoft.com>, "bp@...en8.de" <bp@...en8.de>,
 "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
 "decui@...rosoft.com" <decui@...rosoft.com>,
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 "nunodasneves@...ux.microsoft.com" <nunodasneves@...ux.microsoft.com>,
 "tglx@...utronix.de" <tglx@...utronix.de>,
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Cc: "apais@...rosoft.com" <apais@...rosoft.com>,
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 "vdso@...bites.dev" <vdso@...bites.dev>
Subject: Re: [PATCH v5 1/5] hyperv: Define struct hv_output_get_vp_registers



On 1/6/2025 9:37 AM, Michael Kelley wrote:
> From: Roman Kisel <romank@...ux.microsoft.com> Sent: Monday, December 30, 2024 10:10 AM
[...]
> 
> These bit field definitions don't look right. We want to "fill up"
> the field size, so that we're explicit about each bit, and not leave
> it to the compiler to add padding (which __packed tells the
> compiler not to do). So in aggregate, the "u64" bit fields should
> account for all 64 bits, but here you account for only 32 bits.
> There are two ways to fix this:
> 
> 		u32 interruption_pending : 1;
> 		u32 interruption_type: 1;
> 		u32 reserved : 30;
> 		u32 error_code;
> Or
> 		u64 interruption_pending : 1;
> 		u64 interruption_type: 1;
> 		u64 reserved : 30;
> 		u64 error_code : 32;
> 
> 
> Nuno -- your thoughts?

Quite a blunder on my side! Thank you very much for your help :)
GDB is my witness:

(gdb) ptype /o union hv_arm64_pending_synthetic_exception_event
/* offset      |    size */  type = union 
hv_arm64_pending_synthetic_exception_event {
/*                    16 */    u64 as_uint64[2];
/*                    13 */    struct {
/*      0: 0   |       4 */        u32 event_pending : 1;
/*      0: 1   |       4 */        u32 event_type : 3;
/*      0: 4   |       4 */        u32 reserved : 4;
/*      1      |       4 */        u32 exception_type;
/*      5      |       8 */        u64 context;

                                    /* total size (bytes):   13 */
                                };

                                /* total size (bytes):   16 */
                              }

which looks messed up to me to put it mildly. Will fix next time.

[...]

> 
> Michael
> 
>> +
>> +/*
>> + * NOTE: Linux helper struct - NOT from Hyper-V code.
>> + * DECLARE_FLEX_ARRAY() needs to be wrapped into
>> + * a structure and have at least one more member besides
>> + * DECLARE_FLEX_ARRAY.
>> + */
>> +struct hv_output_get_vp_registers {
>> +	struct {
>> +		DECLARE_FLEX_ARRAY(union hv_register_value, values);
>> +		struct {} values_end;
>> +	};
>>   };
>>
>>   #if defined(CONFIG_ARM64)
>> --
>> 2.34.1

-- 
Thank you,
Roman