linux-kernel - Re: [PATCH] mshv: Make MSHV mutually exclusive with KEXEC

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Message-ID: <2b42997d-7cc0-56ba-e1ca-a8640ce71ea9@linux.microsoft.com>
Date: Mon, 26 Jan 2026 15:07:18 -0800
From: Mukesh R <mrathor@...ux.microsoft.com>
To: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
Cc: kys@...rosoft.com, haiyangz@...rosoft.com, wei.liu@...nel.org,
 decui@...rosoft.com, longli@...rosoft.com, linux-hyperv@...r.kernel.org,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mshv: Make MSHV mutually exclusive with KEXEC

On 1/26/26 12:43, Stanislav Kinsburskii wrote:
> On Mon, Jan 26, 2026 at 12:20:09PM -0800, Mukesh R wrote:
>> On 1/25/26 14:39, Stanislav Kinsburskii wrote:
>>> On Fri, Jan 23, 2026 at 04:16:33PM -0800, Mukesh R wrote:
>>>> On 1/23/26 14:20, Stanislav Kinsburskii wrote:
>>>>> The MSHV driver deposits kernel-allocated pages to the hypervisor during
>>>>> runtime and never withdraws them. This creates a fundamental incompatibility
>>>>> with KEXEC, as these deposited pages remain unavailable to the new kernel
>>>>> loaded via KEXEC, leading to potential system crashes upon kernel accessing
>>>>> hypervisor deposited pages.
>>>>>
>>>>> Make MSHV mutually exclusive with KEXEC until proper page lifecycle
>>>>> management is implemented.
>>>>>
>>>>> Signed-off-by: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
>>>>> ---
>>>>>     drivers/hv/Kconfig |    1 +
>>>>>     1 file changed, 1 insertion(+)
>>>>>
>>>>> diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig
>>>>> index 7937ac0cbd0f..cfd4501db0fa 100644
>>>>> --- a/drivers/hv/Kconfig
>>>>> +++ b/drivers/hv/Kconfig
>>>>> @@ -74,6 +74,7 @@ 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 !KEXEC
>>>>>     	select EVENTFD
>>>>>     	select VIRT_XFER_TO_GUEST_WORK
>>>>>     	select HMM_MIRROR
>>>>>
>>>>>
>>>>
>>>> Will this affect CRASH kexec? I see few CONFIG_CRASH_DUMP in kexec.c
>>>> implying that crash dump might be involved. Or did you test kdump
>>>> and it was fine?
>>>>
>>>
>>> Yes, it will. Crash kexec depends on normal kexec functionality, so it
>>> will be affected as well.
>>
>> So not sure I understand the reason for this patch. We can just block
>> kexec if there are any VMs running, right? Doing this would mean any
>> further developement would be without a ver important and major feature,
>> right?
> 
> This is an option. But until it's implemented and merged, a user mshv
> driver gets into a situation where kexec is broken in a non-obvious way.
> The system may crash at any time after kexec, depending on whether the
> new kernel touches the pages deposited to hypervisor or not. This is a
> bad user experience.

I understand that. But with this we cannot collect core and debug any
crashes. I was thinking there would be a quick way to prohibit kexec
for update via notifier or some other quick hack. Did you already
explore that and didn't find anything, hence this?

Thanks,
-Mukesh

> Therefor it should be explicitly forbidden as it's essentially not
> supported yet.
> 
> Thanks,
> Stanislav
> 
>>
>>> Thanks,
>>> Stanislav
>>>
>>>> Thanks,
>>>> -Mukesh