linux-kernel - RE: [PATCH hyperv-next v5 03/11] Drivers: hv: Enable VTL mode for arm64

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Message-ID:
 <SN6PR02MB4157A635B0D1B43A2ED2664DD4D32@SN6PR02MB4157.namprd02.prod.outlook.com>
Date: Thu, 13 Mar 2025 05:10:10 +0000
From: Michael Kelley <mhklinux@...look.com>
To: Roman Kisel <romank@...ux.microsoft.com>, Arnd Bergmann <arnd@...db.de>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>, Borislav Petkov <bp@...en8.de>,
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 Anvin" <hpa@...or.com>, Joey Gouly <joey.gouly@....com>, "krzk+dt@...nel.org"
	<krzk+dt@...nel.org>, Krzysztof Wilczyński <kw@...ux.com>,
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	<linux-arm-kernel@...ts.infradead.org>, "linux-hyperv@...r.kernel.org"
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CC: "apais@...rosoft.com" <apais@...rosoft.com>, "benhill@...rosoft.com"
	<benhill@...rosoft.com>, "bperkins@...rosoft.com" <bperkins@...rosoft.com>,
	"sunilmut@...rosoft.com" <sunilmut@...rosoft.com>
Subject: RE: [PATCH hyperv-next v5 03/11] Drivers: hv: Enable VTL mode for
 arm64

From: Roman Kisel <romank@...ux.microsoft.com> Sent: Wednesday, March 12, 2025 2:21 PM
> 
> On 3/12/2025 1:25 PM, Arnd Bergmann wrote:
> > On Wed, Mar 12, 2025, at 19:33, Roman Kisel wrote:
> >>
> >> That's a minimal extension, its surprise factor is very low. It has not
> >> been seen to cause issues. If no one has strong opinions against that,
> >> I'd send that in V6.
> >>
> >
> > Works for me. Thanks for your detailed explanations.
> >
> 
> Thank you for your review very much!
> 

My original concern [1] with this minimal change is that it allows building
a normal Linux kernel (i.e., not for VTL 2) for Hyper-V with CONFIG_ACPI=n.
Such a kernel will not run in a Hyper-V VM since ACPI is required unless
building for and running in VTL 2. Current upstream code disallows
CONFIG_HYPERV=y with CONFIG_ACPI=n.

However, I don't want to make too big of a deal about now allowing this
misconfiguration. Arguably it's not likely to happen, and the solution is
"don't do that".  So if we want to go back to the minimal set of changes to
drivers/hv/Kconfig as Roman proposes, I won't object further. I just want
to sure everyone is clear on the tradeoffs.

Michael

[1] https://lore.kernel.org/linux-hyperv/SN6PR02MB4157E15EFE263BBA3D8DFC51D4EC2@SN6PR02MB4157.namprd02.prod.outlook.com/