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

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Message-ID: <48d3a3e0-8e2f-4eb8-b725-b6eed37c290c@linux.microsoft.com>
Date: Thu, 19 Dec 2024 11:13:21 -0800
From: Easwar Hariharan <eahariha@...ux.microsoft.com>
To: Nuno Das Neves <nunodasneves@...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, eahariha@...ux.microsoft.com,
 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 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
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.

- Easwar