Re: [PATCH] kernel config: don't automatically enable kexec handover (KHO)

[Dominik Csapak <d.csapak@proxmox.com>]

just a note for this (maybe can be squashed into the commit message
on applying):

this does not disable the kexec handoff completely, it just
does not enable it by default, so users that would need it
can still enable it via 'kho=on' on the commandline.

conversely, users running into problem with the current configuration
can mitigate the issue by adding 'kho=off' to their commandline
to disable it.

On 7/14/26 2:44 PM, Dominik Csapak wrote:
> CONFIG_KEXEC_HANDOVER_ENABLE_DEFAULT is enabled by default in the
> upstream ubuntu kernel config, but has a (probably) unintended
> side-effect: when KHO is automatically enabled, the kernel reserves
> scratch memory for the kernel handoff and marks it with MIGRATE_CMA to
> prevent non-movable allocations there.
> 
> This causes unusual stats in /proc/meminfo, for example:
> 
> CmaTotal:              0 kB
> CmaFree:         4894720 kB
> 
> which looks nonsensical on the surface. It also reduces the available
> memory for e.g. guests with PCI passthrough, as pinner memory can't live
> in these areas.
> 
> Since we don't use kexec handover by default, and it can still be
> enabled on the kernel commandline (kho=on), disable it by default to
> increase the available pinnable memory for guests.
> 
> Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
> ---
>   debian/rules.d/config-common.opts | 1 +
>   1 file changed, 1 insertion(+)
> 
> diff --git a/debian/rules.d/config-common.opts b/debian/rules.d/config-common.opts
> index 64505b4..204bab4 100644
> --- a/debian/rules.d/config-common.opts
> +++ b/debian/rules.d/config-common.opts
> @@ -68,3 +68,4 @@
>   -d CONFIG_N_GSM
>   -d UBSAN_BOUNDS
>   -e CONFIG_RUST
> +-d CONFIG_KEXEC_HANDOVER_ENABLE_DEFAULT