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Date:   Tue, 7 Sep 2021 14:34:57 -0400
From:   Eduardo Habkost <ehabkost@redhat.com>
To:     Juergen Gross <jgross@suse.com>
Cc:     kvm@vger.kernel.org, x86@kernel.org, linux-kernel@vger.kernel.org,
        maz@kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH v2 3/6] x86/kvm: introduce per cpu vcpu masks
Message-ID: <20210907183457.53ws6tqqqpzkeil4@habkost.net>
References: <20210903130808.30142-1-jgross@suse.com>
 <20210903130808.30142-4-jgross@suse.com>
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On Fri, Sep 03, 2021 at 03:08:04PM +0200, Juergen Gross wrote:
> In order to support high vcpu numbers per guest don't use on stack
> vcpu bitmasks. As all those currently used bitmasks are not used in
> functions subject to recursion it is fairly easy to replace them with
> percpu bitmasks.
> 
> Disable preemption while such a bitmask is being used in order to
> avoid double usage in case we'd switch cpus.
> 
> Signed-off-by: Juergen Gross <jgross@suse.com>
> ---
> V2:
> - use local_lock() instead of preempt_disable() (Paolo Bonzini)
> ---
>  arch/x86/include/asm/kvm_host.h | 10 ++++++++++
>  arch/x86/kvm/hyperv.c           | 25 ++++++++++++++++++-------
>  arch/x86/kvm/irq_comm.c         |  9 +++++++--
>  arch/x86/kvm/x86.c              | 22 +++++++++++++++++++++-
>  4 files changed, 56 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 3513edee8e22..a809a9e4fa5c 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -15,6 +15,7 @@
>  #include <linux/cpumask.h>
>  #include <linux/irq_work.h>
>  #include <linux/irq.h>
> +#include <linux/local_lock.h>
>  
>  #include <linux/kvm.h>
>  #include <linux/kvm_para.h>
> @@ -1591,6 +1592,15 @@ extern bool kvm_has_bus_lock_exit;
>  /* maximum vcpu-id */
>  unsigned int kvm_max_vcpu_id(void);
>  
> +/* per cpu vcpu bitmasks, protected by kvm_pcpu_mask_lock */
> +DECLARE_PER_CPU(local_lock_t, kvm_pcpu_mask_lock);
> +extern unsigned long __percpu *kvm_pcpu_vcpu_mask;
> +#define KVM_VCPU_MASK_SZ	\
> +	(sizeof(*kvm_pcpu_vcpu_mask) * BITS_TO_LONGS(KVM_MAX_VCPUS))
> +extern u64 __percpu *kvm_hv_vp_bitmap;
> +#define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64)
> +#define KVM_HV_VPMAP_SZ		(sizeof(u64) * KVM_HV_MAX_SPARSE_VCPU_SET_BITS)

I have just realized that the Hyper-V sparse bitmap format can
support only up to 4096 CPUs, and the current implementation of
sparse_set_to_vcpu_mask() won't even work correctly if
KVM_MAX_VCPUS is larger than 4096.

This means vp_bitmap can't and will never be larger than 512
bytes.  Isn't a per-CPU variable for vp_bitmap overkill in this
case?

> [...]

-- 
Eduardo