On Tue, Jan 27, 2026 at 11:56:02AM -0800, Mukesh R wrote: > On 1/27/26 09:47, Stanislav Kinsburskii wrote: > > On Mon, Jan 26, 2026 at 05:39:49PM -0800, Mukesh R wrote: > > > On 1/26/26 16:21, Stanislav Kinsburskii wrote: > > > > On Mon, Jan 26, 2026 at 03:07:18PM -0800, Mukesh R wrote: > > > > > 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 > > > > > > > > > > <[email protected]> > > > > > > > > > > --- > > > > > > > > > > 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? > > > > > > > > > > > > > This quick hack you mention isn't quick in the upstream kernel as there > > > > is no hook to interrupt kexec process except the live update one. > > > > > > That's the one we want to interrupt and block right? crash kexec > > > is ok and should be allowed. We can document we don't support kexec > > > for update for now. > > > > > > > I sent an RFC for that one but given todays conversation details is > > > > won't be accepted as is. > > > > > > Are you taking about this? > > > > > > "mshv: Add kexec safety for deposited pages" > > > > > > > Yes. > > > > > > Making mshv mutually exclusive with kexec is the only viable option for > > > > now given time constraints. > > > > It is intended to be replaced with proper page lifecycle management in > > > > the future. > > > > > > Yeah, that could take a long time and imo we cannot just disable KEXEC > > > completely. What we want is just block kexec for updates from some > > > mshv file for now, we an print during boot that kexec for updates is > > > not supported on mshv. Hope that makes sense. > > > > > > > The trade-off here is between disabling kexec support and having the > > kernel crash after kexec in a non-obvious way. This affects both regular > > kexec and crash kexec. > > crash kexec on baremetal is not affected, hence disabling that > doesn't make sense as we can't debug crashes then on bm. >
Bare metal support is not currently relevant, as it is not available. This is the upstream kernel, and this driver will be accessible to third-party customers beginning with kernel 6.19 for running their kernels in Azure L1VH, so consistency is required. Thanks, Stanislav > Let me think and explore a bit, and if I come up with something, I'll > send a patch here. If nothing, then we can do this as last resort. > > Thanks, > -Mukesh > > > > It?s a pity we can?t apply a quick hack to disable only regular kexec. > > However, since crash kexec would hit the same issues, until we have a > > proper state transition for deposted pages, the best workaround for now > > is to reset the hypervisor state on every kexec, which needs design, > > work, and testing. > > > > Disabling kexec is the only consistent way to handle this in the > > upstream kernel at the moment. > > > > Thanks, Stanislav > > > > > > > Thanks, > > > -Mukesh > > > > > > > > > > > > > Thanks, > > > > Stanislav > > > > > > > > > Thanks, > > > > > -Mukesh > > > > > > > > > > > Therefor it should be explicitly forbidden as it's essentially not > > > > > > supported yet. > > > > > > > > > > > > Thanks, > > > > > > Stanislav > > > > > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > Stanislav > > > > > > > > > > > > > > > > > Thanks, > > > > > > > > > -Mukesh
