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Date:   Wed, 25 Jan 2017 14:26:21 -0500
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Joao Martins <joao.m.martins@...cle.com>,
        linux-kernel@...r.kernel.org, xen-devel@...ts.xenproject.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Juergen Gross <jgross@...e.com>,
        Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH v1 2/3] x86/xen/time: setup vcpu 0 time info page

On 01/25/2017 12:33 PM, 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@...cle.com>
> ---
> 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/enlighten.c     |  2 ++
>  arch/x86/xen/time.c          | 51 ++++++++++++++++++++++++++++++++++++++++++++
>  arch/x86/xen/xen-ops.h       |  1 +
>  include/xen/interface/vcpu.h | 28 ++++++++++++++++++++++++
>  4 files changed, 82 insertions(+)
>
> diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
> index 51ef952..15d271d 100644
> --- a/arch/x86/xen/enlighten.c
> +++ b/arch/x86/xen/enlighten.c
> @@ -270,6 +270,8 @@ void xen_vcpu_restore(void)

This is called for PV only. What about HVM?

>  		    HYPERVISOR_vcpu_op(VCPUOP_up, xen_vcpu_nr(cpu), NULL))
>  			BUG();
>  	}
> +
> +	xen_setup_vsyscall_time_info(0);

Do we need to tear down time memory area on VCPU suspend?


>  }
>  
>  static void __init xen_banner(void)
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index 1e69956..e90f703 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -367,6 +367,56 @@ static const struct pv_time_ops xen_time_ops __initconst = {
>  	.steal_clock = xen_steal_clock,
>  };
>  
> +int xen_setup_vsyscall_time_info(int cpu)
> +{
> +	struct pvclock_vsyscall_time_info *xen_clock;
> +	struct vcpu_register_time_memory_area t;
> +	struct pvclock_vcpu_time_info *pvti;
> +	unsigned long addr;
> +	u8 flags;
> +	int ret;
> +
> +	addr = get_zeroed_page(GFP_KERNEL);
> +	if (!addr)
> +		return -ENOMEM;
> +
> +	xen_clock = (struct pvclock_vsyscall_time_info *) addr;
> +	memset(xen_clock, 0, PAGE_SIZE);

You don't really need addr and there is no reason to memset the page to
zero, given that you got it with get_zeroed_page().

> +
> +	t.addr.v = &xen_clock->pvti;
> +
> +	ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area,
> +				 cpu, &t);
> +
> +	if (ret) {
> +		pr_debug("xen: cannot register vcpu_time_info err %d\n", ret);

pr_warn() would be more appropriate I think. You also have blank line
before 'if'.

> +		free_page(addr);
> +		return ret;
> +	}
> +
> +	pvti = &xen_clock->pvti;
> +	flags = pvti->flags;

I don't think you need these, given that you only reference flags once
below.

> +
> +	if (!(flags & PVCLOCK_TSC_STABLE_BIT)) {
> +		t.addr.v = NULL;
> +		ret = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_time_memory_area,
> +					 cpu, &t);
> +		if (!ret)
> +			free_page(addr);
> +
> +		WARN_ON(ret);

Did someone ask for WARN_ON()? (from your changelog it looks like it
could have been either David or me). I think pr_warn() is sufficient,
just like above.

> +		pr_debug("xen: VCLOCK_PVCLOCK not supported\n");

pr_notice()

-boris

> +		return -ENOTSUPP;
> +	}
> +
> +	pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
> +	pvclock_set_pvti_cpu0_va(xen_clock);
> +
> +	xen_clocksource.archdata.vclock_mode = VCLOCK_PVCLOCK;
> +
> +	return 0;
> +}
> +
>  static void __init xen_time_init(void)
>  {
>  	int cpu = smp_processor_id();
> @@ -393,6 +443,7 @@ static void __init xen_time_init(void)
>  	setup_force_cpu_cap(X86_FEATURE_TSC);
>  
>  	xen_setup_runstate_info(cpu);
> +	xen_setup_vsyscall_time_info(cpu);
>  	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 ac0a2b0..4036d15 100644
> --- a/arch/x86/xen/xen-ops.h
> +++ b/arch/x86/xen/xen-ops.h
> @@ -66,6 +66,7 @@ void __init xen_vmalloc_p2m_tree(void);
>  void xen_init_irq_ops(void);
>  void xen_setup_timer(int cpu);
>  void xen_setup_runstate_info(int cpu);
> +int xen_setup_vsyscall_time_info(int cpu);
>  void xen_teardown_timer(int cpu);
>  u64 xen_clocksource_read(void);
>  void xen_setup_cpu_clockevents(void);
> diff --git a/include/xen/interface/vcpu.h b/include/xen/interface/vcpu.h
> index 98188c8..8da788c 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);
> +
>  #endif /* __XEN_PUBLIC_VCPU_H__ */


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