Re: [PATCH] x86/shadow: Delete the none.c dummy file

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Feb 09, 2026 at 11:41:02AM +0100, Alejandro Vallejo wrote:
> It only has 2 callers, both of which can be conditionally removed.
> 
> Signed-off-by: Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>
> ---
> I'd be ok conditionalising the else branch on...
> 
>     IS_ENABLED(CONFIG_SHADOW_PAGING )|| IS_ENABLED(CONFIG_LOG_DIRTY)
> 
> logdirty patch: 
> https://lore.kernel.org/xen-devel/20260209103118.5885-1-alejandro.garciavallejo@xxxxxxx
> 
> ... to avoid the danger of stale pointers, with required changes elsewhere so
> none.c is only compiled out in that case.
> 
> I'm not sure how much it matters seeing how they are all unreachable.
> ---
>  xen/arch/x86/mm/Makefile        |  2 +-
>  xen/arch/x86/mm/paging.c        |  4 +-
>  xen/arch/x86/mm/shadow/Makefile |  4 --
>  xen/arch/x86/mm/shadow/none.c   | 77 ---------------------------------
>  4 files changed, 3 insertions(+), 84 deletions(-)
>  delete mode 100644 xen/arch/x86/mm/shadow/none.c
> 
> diff --git a/xen/arch/x86/mm/Makefile b/xen/arch/x86/mm/Makefile
> index 960f6e8409..066c4caff3 100644
> --- a/xen/arch/x86/mm/Makefile
> +++ b/xen/arch/x86/mm/Makefile
> @@ -1,4 +1,4 @@
> -obj-y += shadow/
> +obj-$(CONFIG_SHADOW_PAGING) += shadow/
>  obj-$(CONFIG_HVM) += hap/
>  
>  obj-$(CONFIG_ALTP2M) += altp2m.o
> diff --git a/xen/arch/x86/mm/paging.c b/xen/arch/x86/mm/paging.c
> index 2396f81ad5..5f70254cec 100644
> --- a/xen/arch/x86/mm/paging.c
> +++ b/xen/arch/x86/mm/paging.c
> @@ -634,7 +634,7 @@ int paging_domain_init(struct domain *d)
>       */
>      if ( hap_enabled(d) )
>          hap_domain_init(d);
> -    else
> +    else if ( IS_ENABLED(CONFIG_SHADOW_PAGING) )
>          rc = shadow_domain_init(d);

If you want to go this route you will need to set rc = -EOPNOTSUPP;
prior to the `if ... else if` on the HVM case.  Otherwise when shadow
is compiled out, and the toolstack has not specified the HAP flag at
domain creation you will end up with a domain that doesn't have the
paging operations initialized as paging_domain_init() would return 0
with neither HAP nor shadow having been setup.  That's likely to
trigger NULL pointer dereferences inside of Xen.

Also, seeing the code in arch_sanitise_domain_config() we possibly
want to return an error at that point if toolstack attempts to create
an HVM guest without HAP enabled, and shadow is build time disabled.
I've sent a patch to that end.

Thanks, Roger.