linux-kernel - Re: [EXTERNAL] RE: [PATCH 0/5] Add new headers for Hyper-V Dom0

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Message-ID: <d70bbad7-bcad-2031-a4e1-755b502422a4@microsoft.com>
Date: Fri, 11 Oct 2024 01:34:52 +0000
From: MUKESH RATHOR <mukeshrathor@...rosoft.com>
To: Michael Kelley <mhklinux@...look.com>, Nuno Das Neves
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Subject: Re: [EXTERNAL] RE: [PATCH 0/5] Add new headers for Hyper-V Dom0



On 10/10/24 11:21, Michael Kelley wrote:
 > From: Nuno Das Neves <nunodasneves@...ux.microsoft.com> Sent: 
Thursday, October 3, 2024 12:51 PM
 >>
 >> To support Hyper-V Dom0 (aka Linux as root partition), many new
 >> definitions are required.
 >>
 >> The plan going forward is to directly import headers from
 >> Hyper-V. This is a more maintainable way to import definitions
 >> rather than via the TLFS doc. This patch series introduces
 >> new headers (hvhdk.h, hvgdk.h, etc, see patch #3) directly
 >> derived from Hyper-V code.
 >>
 >> This patch series replaces hyperv-tlfs.h with hvhdk.h, but only
 >> in Microsoft-maintained Hyper-V code where they are needed. This
 >> leaves the existing hyperv-tlfs.h in use elsewhere - notably for
 >> Hyper-V enlightenments on KVM guests.
 >
 > Could you elaborate on why the bifurcation is necessary? Is it an
 > interim step until the KVM code can use the new scheme as well?
 > Also, does "Hyper-V enlightenments on KVM guests" refer to
 > nested KVM running at L1 on an L0 Hyper-V, and supporting L2 guests?
 > Or is it the more general KVM support for mimicking Hyper-V for
 > the purposes of running Windows guests? From these patches, it
 > looks like your intention is for all KVM support for Hyper-V
 > functionality to continue to use the existing hyperv-tlfs.h file.

Like it says above, we are creating new dom0 (root/host) support
that requires many new defs only available to dom0 and not any
guest. Hypervisor makes them publicly available via hv*dk files.

Ideally, someday everybody will use those, I hope we can move in
that direction, but I guess one step at a time. For now, KVM can
continue to use the tlfs file, and if there is no resistance, we
can move them to hv*dk files also as next step and obsolete the
single tlfs file.

Since headers are the ultimate source of truth, this will allow
better maintenance, better debug/support experience, and a more
stable stack. It also enforces non-leaking of data structs from
private header files (unfortunately has happened).

Thanks
-Mukesh