Re: [PATCH v2 1/1] xen: update system time immediately when VCPUOP_register_vcpu_info

[Stefano Stabellini <sstabellini@kernel.org>]

On Mon, 25 Oct 2021, Dongli Zhang wrote:
> The guest may access the pv vcpu_time_info immediately after
> VCPUOP_register_vcpu_info. This is to borrow the idea of
> VCPUOP_register_vcpu_time_memory_area, where the
> force_update_vcpu_system_time() is called immediately when the new memory
> area is registered.
> 
> Otherwise, we may observe clock drift at the VM side if the VM accesses
> the clocksource immediately after VCPUOP_register_vcpu_info().
> 
> Reference:
> https://lists.xenproject.org/archives/html/xen-devel/2021-10/msg00571.html
> Cc: Joe Jin <joe.jin@oracle.com>
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> ---
> Changed since v1:
>   - Implement force_update_vcpu_system_time() for ARM
>     (suggested by Jan Beulich)
>     While I have tested ARM compilation with aarch64-linux-gnu-gcc, I did
>     not test on ARM platform.

This is fine. Of course it is not going to do anything on ARM given that
update_vcpu_system_time in unimplemented but it is certainly harmless.

For the ARM part:
Acked-by: Stefano Stabellini <sstabellini@kernel.org>


>  xen/arch/arm/time.c        | 5 +++++
>  xen/common/domain.c        | 2 ++
>  xen/include/asm-arm/time.h | 2 ++
>  3 files changed, 9 insertions(+)
> 
> diff --git a/xen/arch/arm/time.c b/xen/arch/arm/time.c
> index 7dbd363537..dec53b5f7d 100644
> --- a/xen/arch/arm/time.c
> +++ b/xen/arch/arm/time.c
> @@ -351,6 +351,11 @@ void update_vcpu_system_time(struct vcpu *v)
>      /* XXX update shared_info->wc_* */
>  }
>  
> +void force_update_vcpu_system_time(struct vcpu *v)
> +{
> +    update_vcpu_system_time(v);
> +}
> +
>  void domain_set_time_offset(struct domain *d, int64_t time_offset_seconds)
>  {
>      d->time_offset.seconds = time_offset_seconds;
> diff --git a/xen/common/domain.c b/xen/common/domain.c
> index 8b53c49d1e..d71fcab88c 100644
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -1704,6 +1704,8 @@ long do_vcpu_op(int cmd, unsigned int vcpuid, XEN_GUEST_HANDLE_PARAM(void) arg)
>          rc = map_vcpu_info(v, info.mfn, info.offset);
>          domain_unlock(d);
>  
> +        force_update_vcpu_system_time(v);
> +
>          break;
>      }
>  
> diff --git a/xen/include/asm-arm/time.h b/xen/include/asm-arm/time.h
> index 6b8fd839dd..4b401c1110 100644
> --- a/xen/include/asm-arm/time.h
> +++ b/xen/include/asm-arm/time.h
> @@ -105,6 +105,8 @@ extern uint64_t ns_to_ticks(s_time_t ns);
>  
>  void preinit_xen_time(void);
>  
> +void force_update_vcpu_system_time(struct vcpu *v);
> +
>  #endif /* __ARM_TIME_H__ */
>  /*
>   * Local variables:
> -- 
> 2.17.1
>