Re: [PATCH] x86/domain: adjust limitation on shared_info allocation below 4G

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

On Tue, Feb 03, 2026 at 11:10:17AM +0100, Roger Pau Monne wrote:
> The limitation of shared_info being allocated below 4G to fit in the
> start_info field only applies to 32bit PV guests.  On 64bit PV guests the
> start_info field is 64bits wide.  HVM guests don't use start_info at all.
> 
> Limit the allocation address restriction to 32bit PV guests only.
> 
> Fixes: 3cadc0469d5c ("x86_64: shared_info must be allocated below 4GB as it 
> is advertised to 32-bit guests via a 32-bit machine address field in 
> start_info.")
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/arch/x86/domain.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index edb76366b596..4163568043b1 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -882,10 +882,13 @@ int arch_domain_create(struct domain *d,
>          goto fail;
>  
>      /*
> -     * The shared_info machine address must fit in a 32-bit field within a
> -     * 32-bit guest's start_info structure. Hence we specify MEMF_bits(32).
> +     * For 32bit PV guests the shared_info machine address must fit in a 
> 32-bit
> +     * field within the guest's start_info structure. Hence we specify
> +     * MEMF_bits(32).
>       */
> -    if ( (d->shared_info = alloc_xenheap_pages(0, MEMF_bits(32))) == NULL )
> +    if ( (d->shared_info =
> +          alloc_xenheap_pages(0, is_pv_32bit_domain(d) ? MEMF_bits(32)
> +                                                       : 0)) == NULL )

Sorry, this is wrong, it's too early to know whether the domain is 32
or 64bit.

Roger.