linux-kernel - Re: [PATCH 1/2] asm-generic/hyperv: provide cpumask_to_vpset

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Date:   Thu, 09 Sep 2021 07:38:28 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Wei Liu <wei.liu@...nel.org>,
        Linux on Hyper-V List <linux-hyperv@...r.kernel.org>
Cc:     Michael Kelley <mikelley@...rosoft.com>, kys@...rosoft.com,
        haiyangz@...rosoft.com, decui@...rosoft.com,
        sthemmin@...rosoft.com, Wei Liu <wei.liu@...nel.org>,
        Arnd Bergmann <arnd@...db.de>,
        "open list:GENERIC INCLUDE/ASM HEADER FILES" 
        <linux-arch@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] asm-generic/hyperv: provide cpumask_to_vpset_noself

Wei Liu <wei.liu@...nel.org> writes:

> This is a new variant which removes `self' cpu from the vpset. It will
> be used in Hyper-V enlightened IPI code.
>
> Signed-off-by: Wei Liu <wei.liu@...nel.org>
> ---
> Provide a new variant instead of adding a new parameter because it makes
> it easier to backport -- we don't need to fix the users of
> cpumask_to_vpset.
> ---
>  include/asm-generic/mshyperv.h | 20 ++++++++++++++++++--
>  1 file changed, 18 insertions(+), 2 deletions(-)
>
> diff --git a/include/asm-generic/mshyperv.h b/include/asm-generic/mshyperv.h
> index 9a000ba2bb75..d89690ee95aa 100644
> --- a/include/asm-generic/mshyperv.h
> +++ b/include/asm-generic/mshyperv.h
> @@ -184,10 +184,12 @@ static inline int hv_cpu_number_to_vp_number(int cpu_number)
>  	return hv_vp_index[cpu_number];
>  }
>  
> -static inline int cpumask_to_vpset(struct hv_vpset *vpset,
> -				    const struct cpumask *cpus)
> +static inline int cpumask_to_vpset_ex(struct hv_vpset *vpset,

I'd suggest to avoid '_ex' suffix as we use it for 'extended hypercalls'
(e.g. __send_ipi_mask_ex). Assuming nobody needs to call
cpumask_to_vpset_ex() directly, should we just go with
__cpumask_to_vpset() instead?

> +				    const struct cpumask *cpus,
> +				    bool exclude_self)
>  {
>  	int cpu, vcpu, vcpu_bank, vcpu_offset, nr_bank = 1;
> +	int this_cpu = smp_processor_id();
>  
>  	/* valid_bank_mask can represent up to 64 banks */
>  	if (hv_max_vp_index / 64 >= 64)
> @@ -205,6 +207,8 @@ static inline int cpumask_to_vpset(struct hv_vpset *vpset,
>  	 * Some banks may end up being empty but this is acceptable.
>  	 */
>  	for_each_cpu(cpu, cpus) {
> +		if (exclude_self && cpu == this_cpu)
> +			continue;
>  		vcpu = hv_cpu_number_to_vp_number(cpu);
>  		if (vcpu == VP_INVAL)
>  			return -1;
> @@ -219,6 +223,18 @@ static inline int cpumask_to_vpset(struct hv_vpset *vpset,
>  	return nr_bank;
>  }
>  
> +static inline int cpumask_to_vpset(struct hv_vpset *vpset,
> +				    const struct cpumask *cpus)
> +{
> +	return cpumask_to_vpset_ex(vpset, cpus, false);
> +}
> +
> +static inline int cpumask_to_vpset_noself(struct hv_vpset *vpset,
> +				    const struct cpumask *cpus)
> +{
> +	return cpumask_to_vpset_ex(vpset, cpus, true);


We need to make sure this is called with preemption disabled. We
could've just swapped smp_processor_id() for get_cpu() in
cpumask_to_vpset_ex() but this is hardly a solution: we can get
preempted right after put_cpu() so it's really the caller of this
function which needs to be protected.

TL;DR: I suggest we add 'WARN_ON_ONCE(preemptible());' or something like
this here to catch wrong usage.

> +}
> +
>  void hyperv_report_panic(struct pt_regs *regs, long err, bool in_die);
>  bool hv_is_hyperv_initialized(void);
>  bool hv_is_hibernation_supported(void);

-- 
Vitaly