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

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Message-ID: <aXfSDm-4BjPPZMNu@skinsburskii.localdomain>
Date: Mon, 26 Jan 2026 12:43:58 -0800
From: Stanislav Kinsburskii <skinsburskii@...ux.microsoft.com>
To: Mukesh R <mrathor@...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 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.
Therefor it should be explicitly forbidden as it's essentially not
supported yet.

Thanks,
Stanislav

> 
> > Thanks,
> > Stanislav
> > 
> > > Thanks,
> > > -Mukesh