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

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

On Wed, Feb 04, 2026 at 09:41:21AM +0100, Andrew Cooper wrote:
> On 03/02/2026 10:20 am, Roger Pau Monné wrote:
> > 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.
> 
> It's probably fine to have this become an unrestricted xenhelp
> allocation, and for switch_compat() to make a restricted allocation and
> copy.

Yeah, that's what I'm doing.  It turns out to be a bit more
complicated that I originally anticipated, because you also need to
reset the v->vcpu_info_area.map filed to point to the new page.

> When constructing a PV32 guest in practice, the set_compat hypercall is
> only moments after from the domain create, and it doesn't matter if we
> discover lowmem exhaustion marginally later
> 
> That way we don't have a PV32-ism continuing to impact the all VMs.

Indeed, that's my approach, just took me a bit more than expected
because of the already taken references to the shared_info page by the
time switch_compat() is executed.

Regards, Roger.