Re: [RFC v4 17/24] hw/arm/virt: Reserve one bit of guest physical address for RME

[Mathieu Poirier <mathieu.poirier@linaro.org>]

On Fri, Oct 02, 2026 at 02:19:11PM +1000, Gavin Shan wrote:
> On 9/4/26 5:36 AM, Mathieu Poirier wrote:
> > From: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > 
> > When RME is enabled, the upper GPA bit is used to distinguish protected
> > from unprotected addresses. Reserve it when setting up the guest memory
> > map.
> > 
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
> > ---
> >   hw/arm/virt.c | 20 +++++++++++++++-----
> >   1 file changed, 15 insertions(+), 5 deletions(-)
> > 
> > diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> > index 61751aa31e0d..cf3a9320dc3a 100644
> > --- a/hw/arm/virt.c
> > +++ b/hw/arm/virt.c
> > @@ -4054,17 +4054,27 @@ static int virt_kvm_type(MachineState *ms, const char *type_str)
> >   {
> >       VirtMachineState *vms = VIRT_MACHINE(ms);
> >       int max_vm_pa_size, requested_pa_size;
> > +    int rme_reserve_bit = 0;
> >       bool fixed_ipa;
> >       int vm_type;
> > -    max_vm_pa_size = kvm_arm_get_max_vm_ipa_size(ms, &fixed_ipa);
> > -
> >       vm_type = (ms->cgs ? KVM_VM_TYPE_ARM_REALM : KVM_VM_TYPE_ARM_NORMAL);
> > +    if (vm_type) {
> > +        /*
> > +         * With RME, the upper GPA bit differentiates Realm from NS memory.
> > +         * Reserve the upper bit to ensure that highmem devices will fit.
> > +         */
> > +        rme_reserve_bit = 1;
> > +    }
> 
> "if (vm_type)" are mapped to any VM types other than KVM_VM_TYPE_ARM_NORMAL, meaning
> it can be PKVM in theory. So this would be:
> 
>        if (vm_type == KVM_VM_TYPE_ARM_REALM) {
>            ...
>        }

I agree.

> 
> > +
> > +    max_vm_pa_size = kvm_arm_get_max_vm_ipa_size(ms, &fixed_ipa) -
> > +                     rme_reserve_bit;
> > +
> >       /* we freeze the memory map to compute the highest gpa */
> >       virt_set_memmap(vms, max_vm_pa_size);
> > -    requested_pa_size = 64 - clz64(vms->highest_gpa);
> > +    requested_pa_size = 64 - clz64(vms->highest_gpa) + rme_reserve_bit;
> >       /*
> >        * KVM requires the IPA size to be at least 32 bits.
> > @@ -4073,11 +4083,11 @@ static int virt_kvm_type(MachineState *ms, const char *type_str)
> >           requested_pa_size = 32;
> >       }
> > -    if (requested_pa_size > max_vm_pa_size) {
> > +    if (requested_pa_size > max_vm_pa_size + rme_reserve_bit) {
> >           error_report("-m and ,maxmem option values "
> >                        "require an IPA range (%d bits) larger than "
> >                        "the one supported by the host (%d bits)",
> > -                     requested_pa_size, max_vm_pa_size);
> > +                     requested_pa_size, max_vm_pa_size + rme_reserve_bit);
> >           return -1;
> >       }
> >       /*
> 
> Thanks,
> Gavin
>