Re: [Xen-devel] [PATCH v2 2/3] x86/xen/time: setup vcpu 0 time info page

[Juergen Gross <jgross@xxxxxxxx>]

On 22/09/17 18:25, Joao Martins wrote:
> In order to support pvclock vdso on xen we need to setup the time
> info page for vcpu 0 and register the page with Xen using the
> VCPUOP_register_vcpu_time_memory_area hypercall. This hypercall
> will also forcefully update the pvti which will set some of the
> necessary flags for vdso. Afterwards we check if it supports the
> PVCLOCK_TSC_STABLE_BIT flag which is mandatory for having
> vdso/vsyscall support. And if so, it will set the cpu 0 pvti that
> will be later on used when mapping the vdso image.
> 
> The xen headers are also updated to include the new hypercall for
> registering the secondary vcpu_time_info struct.
> 
> Signed-off-by: Joao Martins <joao.m.martins@xxxxxxxxxx>
> ---
> Changes since v1:
>  * Check flags ahead to see if the  primary clock can use
>  PVCLOCK_TSC_STABLE_BIT even if secondary registration fails.
> 
>  (Comments from Boris)
>  * Remove addr, addr variables;
>  * Change first pr_debug to pr_warn;
>  * Change last pr_debug to pr_notice;
>  * Add routine to solely register secondary time info.
>  * Move xen_clock to outside xen_setup_vsyscall_time_info to allow
>  restore path to simply re-register secondary time info. Let us
>  handle the restore path more gracefully without re-allocating a
>  page.
>  * Removed cpu argument from xen_setup_vsyscall_time_info()
>  * Adjustment failed registration error messages/loglevel to be the same
>  * Also teardown secondary time info on suspend
> 
> Changes since RFC:
>  (Comments from Boris and David)
>  * Remove Kconfig option
>  * Use get_zeroed_page/free/page
>  * Remove the hypercall availability check
>  * Unregister pvti with arg.addr.v = NULL if stable bit isn't supported.
>  (New)
>  * Set secondary copy on restore such that it works on migration.
>  * Drop global xen_clock variable and stash it locally on
>  xen_setup_vsyscall_time_info.
>  * WARN_ON(ret) if we fail to unregister the pvti.
> ---
>  arch/x86/xen/suspend.c       |   4 ++
>  arch/x86/xen/time.c          | 100 
> +++++++++++++++++++++++++++++++++++++++++++
>  arch/x86/xen/xen-ops.h       |   2 +
>  include/xen/interface/vcpu.h |  28 ++++++++++++
>  4 files changed, 134 insertions(+)
> 
> diff --git a/arch/x86/xen/suspend.c b/arch/x86/xen/suspend.c
> index d6b1680693a9..800ed36ecfba 100644
> --- a/arch/x86/xen/suspend.c
> +++ b/arch/x86/xen/suspend.c
> @@ -16,6 +16,8 @@
>  
>  void xen_arch_pre_suspend(void)
>  {
> +     xen_save_time_memory_area();
> +
>       if (xen_pv_domain())
>               xen_pv_pre_suspend();
>  }
> @@ -26,6 +28,8 @@ void xen_arch_post_suspend(int cancelled)
>               xen_pv_post_suspend(cancelled);
>       else
>               xen_hvm_post_suspend(cancelled);
> +
> +     xen_restore_time_memory_area();
>  }
>  
>  static void xen_vcpu_notify_restore(void *data)
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index 1ecb05db3632..2924b97691c6 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -370,6 +370,105 @@ static const struct pv_time_ops xen_time_ops 
> __initconst = {
>       .steal_clock = xen_steal_clock,
>  };
>  
> +static struct pvclock_vsyscall_time_info *xen_clock __read_mostly;
> +
> +void xen_save_time_memory_area(void)
> +{
> +     struct vcpu_register_time_memory_area t;
> +     int ret;
> +
> +     if (!xen_clock)
> +             return;
> +
> +     t.addr.v = NULL;
> +
> +     ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area, 0, &t);
> +     if (ret != 0)
> +             pr_notice("Cannot save secondary vcpu_time_info (err %d)",
> +                       ret);
> +     else
> +             clear_page(xen_clock);
> +}
> +
> +void xen_restore_time_memory_area(void)
> +{
> +     struct vcpu_register_time_memory_area t;
> +     int ret;
> +
> +     if (!xen_clock)
> +             return;
> +
> +     t.addr.v = &xen_clock->pvti;
> +
> +     ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area, 0, &t);
> +
> +     /*
> +      * We don't disable VCLOCK_PVCLOCK entirely if it fails to register the
> +      * secondary time info with Xen or if we migrated to a host without the
> +      * necessary flags. On both of these cases what happens is either
> +      * process seeing a zeroed out pvti or seeing no PVCLOCK_TSC_STABLE_BIT
> +      * bit set. Userspace checks the latter and if 0, it discards the data
> +      * in pvti and fallbacks to a system call for a reliable timestamp.
> +      */
> +     if (ret != 0)
> +             pr_notice("Cannot restore secondary vcpu_time_info (err %d)",
> +                       ret);
> +}
> +
> +static void xen_setup_vsyscall_time_info(void)
> +{
> +     struct vcpu_register_time_memory_area t;
> +     struct pvclock_vsyscall_time_info *ti;
> +     struct pvclock_vcpu_time_info *pvti;
> +     int ret;
> +
> +     pvti = &__this_cpu_read(xen_vcpu)->time;
> +
> +     /*
> +      * We check ahead on the primary time info if this
> +      * bit is supported hence speeding up Xen clocksource.
> +      */
> +     if (!(pvti->flags & PVCLOCK_TSC_STABLE_BIT))
> +             return;
> +
> +     pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
> +
> +     ti = (struct pvclock_vsyscall_time_info *) get_zeroed_page(GFP_KERNEL);

Coding style: omit the blank after the cast.

> +     if (!ti)
> +             return;
> +
> +     t.addr.v = &ti->pvti;
> +
> +     ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area, 0, &t);
> +     if (ret) {
> +             pr_notice("xen: VCLOCK_PVCLOCK not supported (err %d)\n", ret);
> +             free_page((unsigned long) ti);

Coding style again, once more below.

> +             return;
> +     }
> +
> +     /*
> +      * If the check above succedded this one should too since it's the
> +      * same data on both primary and secondary time infos just different
> +      * memory regions. But we still check it in case hypervisor is buggy.
> +      */
> +     pvti = &ti->pvti;
> +     if (!(pvti->flags & PVCLOCK_TSC_STABLE_BIT)) {
> +             t.addr.v = NULL;
> +             ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area,
> +                                      0, &t);
> +             if (!ret)
> +                     free_page((unsigned long) ti);
> +
> +             pr_notice("xen: VCLOCK_PVCLOCK not supported (err %d)\n", ret);

Mind making the message more descriptive? E.g. instead of reporting
"(err 0)" just telling "(tsc unstable)"?

> +             return;
> +     }
> +
> +     xen_clock = ti;
> +     pvclock_set_pvti_cpu0_va(xen_clock);
> +
> +     xen_clocksource.archdata.vclock_mode = VCLOCK_PVCLOCK;
> +}
> +
>  static void __init xen_time_init(void)
>  {
>       int cpu = smp_processor_id();
> @@ -396,6 +495,7 @@ static void __init xen_time_init(void)
>       setup_force_cpu_cap(X86_FEATURE_TSC);
>  
>       xen_setup_runstate_info(cpu);
> +     xen_setup_vsyscall_time_info();
>       xen_setup_timer(cpu);
>       xen_setup_cpu_clockevents();
>  
> diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
> index c8a6d224f7ed..f96dbedb33d4 100644
> --- a/arch/x86/xen/xen-ops.h
> +++ b/arch/x86/xen/xen-ops.h
> @@ -69,6 +69,8 @@ void xen_setup_runstate_info(int cpu);
>  void xen_teardown_timer(int cpu);
>  u64 xen_clocksource_read(void);
>  void xen_setup_cpu_clockevents(void);
> +void xen_save_time_memory_area(void);
> +void xen_restore_time_memory_area(void);
>  void __init xen_init_time_ops(void);
>  void __init xen_hvm_init_time_ops(void);
>  
> diff --git a/include/xen/interface/vcpu.h b/include/xen/interface/vcpu.h
> index 98188c87f5c1..8da788c5bd4f 100644
> --- a/include/xen/interface/vcpu.h
> +++ b/include/xen/interface/vcpu.h
> @@ -178,4 +178,32 @@ DEFINE_GUEST_HANDLE_STRUCT(vcpu_register_vcpu_info);
>  
>  /* Send an NMI to the specified VCPU. @extra_arg == NULL. */
>  #define VCPUOP_send_nmi             11
> +
> +/*
> + * Register a memory location to get a secondary copy of the vcpu time
> + * parameters.  The master copy still exists as part of the vcpu shared
> + * memory area, and this secondary copy is updated whenever the master copy
> + * is updated (and using the same versioning scheme for synchronisation).
> + *
> + * The intent is that this copy may be mapped (RO) into userspace so
> + * that usermode can compute system time using the time info and the
> + * tsc.  Usermode will see an array of vcpu_time_info structures, one
> + * for each vcpu, and choose the right one by an existing mechanism
> + * which allows it to get the current vcpu number (such as via a
> + * segment limit).  It can then apply the normal algorithm to compute
> + * system time from the tsc.
> + *
> + * @extra_arg == pointer to vcpu_register_time_info_memory_area structure.
> + */
> +#define VCPUOP_register_vcpu_time_memory_area   13
> +DEFINE_GUEST_HANDLE_STRUCT(vcpu_time_info_t);
> +struct vcpu_register_time_memory_area {
> +     union {
> +             GUEST_HANDLE(vcpu_time_info_t) h;
> +             struct pvclock_vcpu_time_info *v;
> +             uint64_t p;
> +     } addr;
> +};
> +DEFINE_GUEST_HANDLE_STRUCT(vcpu_register_time_memory_area_t);
> +

Instead of adding only the operation you need, maybe you could sync just
the complete header from Xen?


Juergen

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxx
https://lists.xen.org/xen-devel