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

[Thomas Lamprecht <t.lamprecht@proxmox.com>]

Am 14.07.26 um 14:44 schrieb Dominik Csapak:
> 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:         -
> 
> 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.

We really should evaluate that though, IMO this [0] is the dream, and
if it works, it might be used more compared to pcie passthrough.

Also, is this actually reserved memory, or just mappings to have a
linear address space available, when needed?

[0]: https://lwn.net/Articles/1033364/

> 
> 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