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Date: Wed, 20 May 2026 10:15:59 -0700 (PDT)
From: Jork Loeser <jloeser@linux.microsoft.com>
To: Arnd Bergmann <arnd@kernel.org>
cc: "K. Y. Srinivasan" <kys@microsoft.com>, 
    Haiyang Zhang <haiyangz@microsoft.com>, Wei Liu <wei.liu@kernel.org>, 
    Dexuan Cui <decui@microsoft.com>, Long Li <longli@microsoft.com>, 
    "Anirudh Rayabharam (Microsoft)" <anirudh@anirudhrb.com>, 
    Stanislav Kinsburskii <skinsburskii@linux.microsoft.com>, 
    Arnd Bergmann <arnd@arndb.de>, linux-hyperv@vger.kernel.org, 
    linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mshv: add vmbus dependency
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On Wed, 20 May 2026, Arnd Bergmann wrote:

> From: Arnd Bergmann <arnd@arndb.de>
>
> When the vmbus driver is not part of the kernel, the mvhv_root
> driver now fails to link:
>
> ERROR: modpost: "hv_vmbus_exists" [drivers/hv/mshv_root.ko] undefined!
>
> Avoid this by adding an explicit Kconfig dependency. Note that
> stubbing out the hv_vmbus_exists() based on configuration would
> also work for some cases, but not with MSHV_ROOT=y and HYPERV_VMBUS=m.
>
> Fixes: f1a9e67c1138 ("mshv: limit SynIC management to MSHV-owned resources")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> ---
> drivers/hv/Kconfig | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
> index 52af086fdeb2..21193b571a80 100644
> --- a/drivers/hv/Kconfig
> +++ b/drivers/hv/Kconfig
> @@ -75,6 +75,7 @@ config MSHV_ROOT
> 	# e.g. When withdrawing memory, the hypervisor gives back 4k pages in
> 	# no particular order, making it impossible to reassemble larger pages
> 	depends on PAGE_SIZE_4KB
> +	depends on HYPERV_VMBUS
> 	select EVENTFD
> 	select VIRT_XFER_TO_GUEST_WORK
> 	select HMM_MIRROR
> -- 
> 2.39.5
>

Yes, this is the right short-term fix. We will need to solve the root case 
(no VMBUS required) with a separate SYNIC driver abstraction.

Reviewed-by: Jork Loeser <jloeser@linux.microsoft.com>

Thank you!
Jork