linux-kernel - Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id bits

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Date:   Wed, 17 Nov 2021 23:44:40 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Juergen Gross <jgross@...e.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH v3 1/4] x86/kvm: add boot parameter for adding vcpu-id
 bits

On Tue, Nov 16, 2021, Juergen Gross wrote:
> diff --git a/arch/x86/kvm/ioapic.c b/arch/x86/kvm/ioapic.c
> index 816a82515dcd..64ba9b1c8b3d 100644
> --- a/arch/x86/kvm/ioapic.c
> +++ b/arch/x86/kvm/ioapic.c
> @@ -685,11 +685,21 @@ static const struct kvm_io_device_ops ioapic_mmio_ops = {
>  int kvm_ioapic_init(struct kvm *kvm)
>  {
>  	struct kvm_ioapic *ioapic;
> +	size_t sz;
>  	int ret;
>  
> -	ioapic = kzalloc(sizeof(struct kvm_ioapic), GFP_KERNEL_ACCOUNT);
> +	sz = sizeof(struct kvm_ioapic) +
> +	     sizeof(*ioapic->rtc_status.dest_map.map) *
> +		    BITS_TO_LONGS(KVM_MAX_VCPU_IDS) +
> +	     sizeof(*ioapic->rtc_status.dest_map.vectors) *
> +		    (KVM_MAX_VCPU_IDS);
> +	ioapic = kzalloc(sz, GFP_KERNEL_ACCOUNT);
>  	if (!ioapic)
>  		return -ENOMEM;
> +	ioapic->rtc_status.dest_map.map = (void *)(ioapic + 1);

Oof.  Just do separate allocations.  I highly doubt the performance of the
emulated RTC hinges on the spatial locality of the bitmap and array.  The array
is going to end up in a second page for most configuration anyways.

> +	ioapic->rtc_status.dest_map.vectors =
> +		(void *)(ioapic->rtc_status.dest_map.map +
> +			 BITS_TO_LONGS(KVM_MAX_VCPU_IDS));
>  	spin_lock_init(&ioapic->lock);
>  	INIT_DELAYED_WORK(&ioapic->eoi_inject, kvm_ioapic_eoi_inject_work);
>  	kvm->arch.vioapic = ioapic;