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

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Message-ID: <aXzmMInsNSvFvBF1@anirudh-surface.localdomain>
Date: Fri, 30 Jan 2026 17:11:12 +0000
From: Anirudh Rayabharam <anirudh@...rudhrb.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 Wed, Jan 28, 2026 at 03:11:14PM -0800, Stanislav Kinsburskii wrote:
> On Wed, Jan 28, 2026 at 04:16:31PM +0000, Anirudh Rayabharam wrote:
> > On Mon, Jan 26, 2026 at 12:46:44PM -0800, Stanislav Kinsburskii wrote:
> > > On Tue, Jan 27, 2026 at 12:19:24AM +0530, Anirudh Rayabharam wrote:
> > > > On Fri, Jan 23, 2026 at 10:20:53PM +0000, 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.
> > > > 
> > > > Someone might want to stop all guest VMs and do a kexec. Which is valid
> > > > and would work without any issue for L1VH.
> > > > 
> > > 
> > > No, it won't work and hypervsisor depostied pages won't be withdrawn.
> > 
> > All pages that were deposited in the context of a guest partition (i.e.
> > with the guest partition ID), would be withdrawn when you kill the VMs,
> > right? What other deposited pages would be left?
> > 
> 
> The driver deposits two types of pages: one for the guests (withdrawn
> upon gust shutdown) and the other - for the host itself (never
> withdrawn).
> See hv_call_create_partition, for example: it deposits pages for the
> host partition.

Hmm.. I see. Is it not possible to reclaim this memory in module_exit?
Also, can't we forcefully kill all running partitions in module_exit and
then reclaim memory? Would this help with kernel consistency
irrespective of userspace behavior?

Thanks,
Anirudh.

> 
> Thanks,
> Stanislav
> 
> > Thanks,
> > Anirudh.
> > 
> > > Also, kernel consisntency must no depend on use space behavior. 
> > > 
> > > > Also, I don't think it is reasonable at all that someone needs to
> > > > disable basic kernel functionality such as kexec in order to use our
> > > > driver.
> > > > 
> > > 
> > > It's a temporary measure until proper page lifecycle management is
> > > supported in the driver.
> > > Mutual exclusion of the driver and kexec is given and thus should be
> > > expclitily stated in the Kconfig.
> > > 
> > > Thanks,
> > > Stanislav
> > > 
> > > > Thanks,
> > > > Anirudh.
> > > > 
> > > > > 
> > > > > 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
> > > > > 
> > > > >