linux-kernel - Re: [PATCH 1/2] hyperv: Fix pointer type for the output of the hypercall in get

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Message-ID: <85d4f360-3dd8-4a2b-bdbe-b1997321f48e@linux.microsoft.com>
Date: Thu, 19 Dec 2024 11:32:15 -0800
From: Easwar Hariharan <eahariha@...ux.microsoft.com>
To: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
Cc: eahariha@...ux.microsoft.com, Roman Kisel <romank@...ux.microsoft.com>,
 hpa@...or.com, kys@...rosoft.com, bp@...en8.de, dave.hansen@...ux.intel.com,
 decui@...rosoft.com, haiyangz@...rosoft.com, mingo@...hat.com,
 mhklinux@...look.com, tglx@...utronix.de, tiala@...rosoft.com,
 wei.liu@...nel.org, linux-hyperv@...r.kernel.org,
 linux-kernel@...r.kernel.org, x86@...nel.org, apais@...rosoft.com,
 benhill@...rosoft.com, ssengar@...rosoft.com, sunilmut@...rosoft.com,
 vdso@...bites.dev
Subject: Re: [PATCH 1/2] hyperv: Fix pointer type for the output of the
 hypercall in get_vtl(void)

On 12/19/2024 11:23 AM, Nuno Das Neves wrote:
> On 12/19/2024 11:13 AM, Easwar Hariharan wrote:
>> On 12/19/2024 10:40 AM, Nuno Das Neves wrote:
>>> On 12/18/2024 12:54 PM, Roman Kisel wrote:
>>>> Commit bc905fa8b633 ("hyperv: Switch from hyperv-tlfs.h to hyperv/hvhdk.h")
>>>> changed the type of the output pointer to `struct hv_register_assoc` from
>>>> `struct hv_get_vp_registers_output`. That leads to an incorrect computation,
>>>> and leaves the system broken.
>>>>
>>> My bad! But, lets not use `struct hv_get_registers_output`. Instead, use
>>> `struct hv_register_value`, since that is the more complete definition of a
>>> register value. The output of the get_vp_registers hypercall is just an array
>>> of these values.
>>>
>>> Ideally we remove `struct hv_get_vp_registers_output` at some point, since
>>> it serves the same role as `struct hv_register_value` but in a more limited
>>> capacity.
>>>
>>> Thanks
>>> Nuno
>>>
>>
>> I had much the same conversation with Roman off-list yesterday.
>>
>> The choice is between using hv_get_registers_output which is clearly the
>> output of the GetVpRegisters hypercall by name, albeit limited as you
> 
> If it's desirable to have a more 'friendly' naming here, then I'd be okay with:
> ```
> /* NOTE: Linux helper struct - NOT from Hyper-V code */
> struct hv_output_get_vp_registers {
> 	DECLARE_FLEX_ARRAY(struct hv_register_value, values);
> }
> ```
> Note also the name is prefixed with "hv_output_" to match other hypercall outputs.

I like the idea of improving our code readability and consistency in
interface naming independently of the hypervisor. That comment should
allow for clarity when new definitions are imported.

> 
>> said, and hv_register_value which is the more complete definition and
>> what the hypervisor actually returns, but does not currently include the
>> arm64 definitions in our copy of hvgdk_mini.h. hv_get_registers_output
>> and hv_register_value overlap in layout for Roman's purposes.
>>
>> FWIW, I'm in favor of adding the arm64 definitions to hv_register_value
>> and using it for this get_vtl() patch.
>>
>> This could be accompanied with migration of hv_get_vpreg128 in arm64/
>> and removal of struct hv_get_registers_output, or that could be deferred
>> to a later patch.
> 
> I'd be happy to submit a followup patch to update the arm64 code to use
> hv_register_value, or a new struct as outlined above.
> 
> It is a pretty small change though, it might be easier to just include it in
> this series.
> 

Thank you! I'll leave it you and Roman to decide how you go about that.

- Easwar