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

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Message-ID: <ed70560d-9901-4859-9bc0-6fa368636438@linux.microsoft.com>
Date: Thu, 19 Dec 2024 11:23:39 -0800
From: Nuno Das Neves <nunodasneves@...ux.microsoft.com>
To: Easwar Hariharan <eahariha@...ux.microsoft.com>
Cc: 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 11:13 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.

> 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.

Nuno

> 
> - Easwar