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

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Message-ID:
 <SN6PR02MB4157A4E18CEDEB19796F21EFD499A@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Thu, 5 Feb 2026 19:09:01 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Jan Kiszka <jan.kiszka@...mens.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

From: Jan Kiszka <jan.kiszka@...mens.com> Sent: Wednesday, February 4, 2026 10:38 PM
> 
> 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...

I just realized that the Hyper-V "Settings" UI for a VM shows COM1 and COM2
only for Gen1 VMs. I don't know why it's not shown for Gen2 VMs. The
Powershell command you found is what I have always used.

Michael