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

[Gavin Shan <gshan@redhat.com>]

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) {
            ...
        }

> +
> +    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