Re: [PATCH 8/8] arm/mpu: Save/restore VTCR_EL2 on vCPU context switch

[Julien Grall <julien@xxxxxxx>]

Hi Luca,

On 20/04/2026 15:25, Luca Fancellu wrote:
> On AArch64 MPU systems, the VTCR register contains the MSA bit that determines
> if the guest is using MPU or MMU at EL1, which is required to be
> saved/restored on vCPU context switch.
>
> On AArch64 MPU systems the VTCR_EL2 register configuration will be saved
> during setup_virt_paging and will be placed in a new member of
> 'struct arch_vcpu' named vtcr_el2, to be retrieved by context switch.
>
> AArch32 MPU systems and MMU systems don't need to save/restore this
> register because they don't have any MSA bit and related functionality,
> so for these architecture a stub is provided.
>
> Signed-off-by: Luca Fancellu <luca.fancellu@xxxxxxx>
> ---
>   xen/arch/arm/arm64/mpu/p2m.c                | 15 +++++++++++++++
>   xen/arch/arm/domain.c                       |  2 ++
>   xen/arch/arm/include/asm/domain.h           |  5 +++++
>   xen/arch/arm/include/asm/mmu/domain-build.h |  2 ++
>   xen/arch/arm/include/asm/mpu/domain-build.h |  6 ++++++
>   xen/arch/arm/include/asm/mpu/p2m.h          | 10 ++++++++++
>   xen/arch/arm/mpu/domain-build.c             | 17 +++++++++++++++++
>   xen/arch/arm/mpu/p2m.c                      |  4 ++++
>   8 files changed, 61 insertions(+)
>
> diff --git a/xen/arch/arm/arm64/mpu/p2m.c b/xen/arch/arm/arm64/mpu/p2m.c
> index a1ec9fcd6195..350cbd7ae94c 100644
> --- a/xen/arch/arm/arm64/mpu/p2m.c
> +++ b/xen/arch/arm/arm64/mpu/p2m.c
> @@ -3,8 +3,12 @@
>   #include <xen/bug.h>
>   #include <xen/init.h>
>   #include <xen/lib.h>
> +#include <xen/sched.h>
>   #include <asm/p2m.h>
>   
> +/* VTCR_EL2 value to be configured for the boot CPU. */
> +register_t __read_mostly vtcr;
You don't seem to use it. Is it necessary? But the name of the global 
variable is a bit too generic. If we want one, it ought to be 
"boot_vtcr". Also doesn't this need to be writeable after boot? If not, 
it should be called "boot_vtcr".
Cheers,

--
Julien Grall