Re: [PATCH v3 17/26] hw/arm/virt: Reserve one bit of guest-physical address for RME

[Gavin Shan <gshan@redhat.com>]

Hi Jean,

On 11/26/24 5:56 AM, Jean-Philippe Brucker wrote:
> 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>
> ---
>   hw/arm/virt.c | 14 ++++++++++++--
>   1 file changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 9836dfbdfb..eb94997914 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -3035,14 +3035,24 @@ static int virt_kvm_type(MachineState *ms, const char *type_str)
>       VirtMachineState *vms = VIRT_MACHINE(ms);
>       int rme_vm_type = kvm_arm_rme_vm_type(ms);
>       int max_vm_pa_size, requested_pa_size;
> +    int rme_reserve_bit = 0;
>       bool fixed_ipa;
>   
> -    max_vm_pa_size = kvm_arm_get_max_vm_ipa_size(ms, &fixed_ipa);
> +    if (rme_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;
> +    }
> +
> +    max_vm_pa_size = kvm_arm_get_max_vm_ipa_size(ms, &fixed_ipa) -
> +                     rme_reserve_bit;
>   

For realm, @max_vm_pa_size is decreased by 1 ...

>       /* 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;
>   

... For realm, @requested_pa_size is increased by 1, meaning there are two bits in
the gap.

>       /*
>        * KVM requires the IPA size to be at least 32 bits.

One bit instead of two bits seems the correct gap for the followup check?

     if (requested_pa_size > max_vm_pa_size) {
         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);
         return -1;
     }

Thanks,
Gavin