linux-kernel - Re: [EXTERNAL] [PATCH] scsi: storvsc: Fix scheduling while atomic on PREEMPT

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Message-ID: <d9f9add2-27e9-4b17-a122-d14918968ea6@siemens.com>
Date: Thu, 5 Feb 2026 07:37:35 +0100
From: Jan Kiszka <jan.kiszka@...mens.com>
To: Michael Kelley <mhklinux@...look.com>, Long Li <longli@...rosoft.com>,
 KY Srinivasan <kys@...rosoft.com>, Haiyang Zhang <haiyangz@...rosoft.com>,
 Wei Liu <wei.liu@...nel.org>, Dexuan Cui <DECUI@...rosoft.com>,
 "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
 "Martin K. Petersen" <martin.petersen@...cle.com>,
 "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>
Cc: "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
 Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
 Florian Bezdeka <florian.bezdeka@...mens.com>,
 RT <linux-rt-users@...r.kernel.org>, Mitchell Levy <levymitchell0@...il.com>
Subject: Re: [EXTERNAL] [PATCH] scsi: storvsc: Fix scheduling while atomic on
 PREEMPT_RT

On 05.02.26 06:42, Michael Kelley wrote:
> From: Jan Kiszka <jan.kiszka@...mens.com> Sent: Monday, February 2, 2026 9:58 PM
>>
>> On 03.02.26 00:47, Long Li wrote:
>>>> From: Jan Kiszka <jan.kiszka@...mens.com>
>>>>
>>>> This resolves the follow splat and lock-up when running with PREEMPT_RT
>>>> enabled on Hyper-V:
>>>
>>> Hi Jan,
>>>
>>> It's interesting to know the use-case of running a RT kernel over Hyper-V.
>>>
>>> Can you give an example?
>>>
>>
>> - functional testing of an RT base image over Hyper-V
>> - re-use of a common RT base image, without exploiting RT properties
>>
>>> As far as I know, Hyper-V makes no RT guarantees of scheduling VPs for a VM.
>>
>> This is well understood and not our goal. We only need the kernel to run
>> correctly over Hyper-V with PREEMPT-RT enabled, and that is not the case
>> right now.
>>
>> Thanks,
>> Jan
>>
>> PS: Who had to idea to drop a virtual UART from Gen 2 VMs? Early boot
>> guest debugging is true fun now...
>>
> 
> Hmmm. I often do printk()-based debugging via a virtual UART in a Gen 2
> VM. The Linux serial console outputs to that virtual UART and I see the
> printk() output in PuTTY on the Windows host. What specifically are you
> trying to do?  I'm trying to remember if there's any unique setup required
> on a Gen 2 VM vs. a Gen 1 VM, and nothing immediately comes to mind.
> Though maybe it's just so baked into my process that I don't remember it!
> 

Indeed:

Powershell> Set-VMComPort -VMName "Debian 13" 1 \\.\pipe\comport

<Start VM>

Powershell> putty -serial \\.\pipe\comport

Well hidden...

Jan

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