linux-kernel - Re: [PART2 PATCH v5 10/12] svm: Introduces AVIC per-VM ID

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Message-ID: <20160812141604.GC22322@potion>
Date:	Fri, 12 Aug 2016 16:16:04 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Suravee Suthikulpanit <suravee.suthikulpanit@....com>
Cc:	joro@...tes.org, pbonzini@...hat.com, alex.williamson@...hat.com,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	sherry.hurwitz@....com
Subject: Re: [PART2 PATCH v5 10/12] svm: Introduces AVIC per-VM ID

2016-07-25 04:32-0500, Suravee Suthikulpanit:
> From: Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>
> 
> Introduces per-VM AVIC ID and helper functions to manage the IDs.
> Currently, the ID will be used to implement 32-bit AVIC IOMMU GA tag.
> 
> The ID is 24-bit one-based indexing value, and is managed via helper
> functions to get the next ID, or to free an ID once a VM is destroyed.
> There should be no ID conflict for any active VMs.
> 
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>
> ---
> diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
> @@ -242,6 +254,10 @@ static int avic;
>  module_param(avic, int, S_IRUGO);
>  #endif
>  
> +/* AVIC VM ID bit masks and lock */
> +static unsigned long *avic_vm_id_bm;

Please expand "bm" to "bitmap" to match KVM conventions.

> +static DEFINE_SPINLOCK(avic_vm_id_lock);
> +
>  static void svm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0);
>  static void svm_flush_tlb(struct kvm_vcpu *vcpu);
>  static void svm_complete_interrupts(struct vcpu_svm *svm);
> @@ -1280,10 +1296,61 @@ static int avic_init_backing_page(struct kvm_vcpu *vcpu)
>  	return 0;
>  }
>  
> +static inline int avic_vm_id_init(void)
> +{
> +	if (avic_vm_id_bm)
> +		return 0;
> +
> +	avic_vm_id_bm = kcalloc(BITS_TO_LONGS(AVIC_VM_ID_MASK),

Allocation is off by one.  avic_get_next_vm_id() uses
  if (id <= AVIC_VM_ID_MASK)
    __set_bit(id, avic_vm_id_bm);

and id=AVIC_VM_ID_MASK is stored in the AVIC_VM_ID_MASK+1 th bit.

> +				sizeof(long), GFP_KERNEL);
> +	if (!avic_vm_id_bm)
> +		return -ENOMEM;
> +	return 0;
> +}
> +
> +static inline void avic_vm_id_deinit(void)
> +{
> +	if (!avic_vm_id_bm)
> +		return;
> +
> +	kfree(avic_vm_id_bm);
> +	avic_vm_id_bm = NULL;
> +}
> +
> +static inline int avic_get_next_vm_id(void)
> +{
> +	int id;
> +
> +	spin_lock(&avic_vm_id_lock);
> +
> +	/* AVIC VM ID is one-based. */

Why?

> +	id = find_next_zero_bit(avic_vm_id_bm, 1, 1);

The second argument is size, so this should always return 1. :)

> +	if (id <= AVIC_VM_ID_MASK)
> +		__set_bit(id, avic_vm_id_bm);
> +	else
> +		id = -EINVAL;

It is not really a problem that can be handled with changing the values,
so a temporary error would be nicer ... ENOMEM could be confusing and
EAGAIN lead to a loop, but I still like them better.

> +
> +	spin_unlock(&avic_vm_id_lock);
> +	return id;
> +}
> +
> +static inline int avic_free_vm_id(int id)
> +{
> +	if (id <= 0 || id > AVIC_VM_ID_MASK)
> +		return -EINVAL;
> +
> +	spin_lock(&avic_vm_id_lock);
> +	__clear_bit(id, avic_vm_id_bm);
> +	spin_unlock(&avic_vm_id_lock);
> +	return 0;
> +}
> +
>  static void avic_vm_destroy(struct kvm *kvm)
>  {
>  	struct kvm_arch *vm_data = &kvm->arch;
>  
> +	avic_free_vm_id(vm_data->avic_vm_id);
> +
>  	if (vm_data->avic_logical_id_table_page)
>  		__free_page(vm_data->avic_logical_id_table_page);
>  	if (vm_data->avic_physical_id_table_page)
> @@ -1300,6 +1367,10 @@ static int avic_vm_init(struct kvm *kvm)
>  	if (!avic)
>  		return 0;
>  
> +	vm_data->avic_vm_id = avic_get_next_vm_id();
> +	if (vm_data->avic_vm_id < 0)
> +		return vm_data->avic_vm_id;
> +
>  	/* Allocating physical APIC ID table (4KB) */
>  	p_page = alloc_page(GFP_KERNEL);
>  	if (!p_page)
> @@ -5076,6 +5147,12 @@ static struct kvm_x86_ops svm_x86_ops = {
>  
>  static int __init svm_init(void)
>  {
> +	int ret;
> +
> +	ret = avic_vm_id_init();

This is certainly useless when the CPU doesn't have AVIC, so we could
make it conditional.

I would prefer to make the bitmap allocated at module load, though:

  static DECLARE_BITMAP(avic_vm_id_bm, AVIC_VM_ID_MASK + 1);

The size is 2 KiB with 24 bit AVIC_VM_ID_MASK, which is IMO much better
than having extra lines of code dealing with allocation and failures.

> +	if (ret)
> +		return ret;
> +
>  	return kvm_init(&svm_x86_ops, sizeof(struct vcpu_svm),
>  			__alignof__(struct vcpu_svm), THIS_MODULE);
>  }