Re: [PATCH v14 6/7] KVM: arm: dirty logging write protect support

[Christoffer Dall <christoffer.dall@linaro.org>]

On Thu, Nov 13, 2014 at 05:57:47PM -0800, Mario Smarduch wrote:
> Add support to track dirty pages between user space KVM_GET_DIRTY_LOG ioctl
> calls. We call kvm_get_dirty_log_protect() function to do most of the work.
> 
> Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
> Signed-off-by: Mario Smarduch <m.smarduch@samsung.com>
> ---
>  arch/arm/kvm/Kconfig |    1 +
>  arch/arm/kvm/arm.c   |   46 ++++++++++++++++++++++++++++++++++++++++++++++
>  arch/arm/kvm/mmu.c   |   22 ++++++++++++++++++++++
>  3 files changed, 69 insertions(+)
> 
> diff --git a/arch/arm/kvm/Kconfig b/arch/arm/kvm/Kconfig
> index f27f336..a8d1ace 100644
> --- a/arch/arm/kvm/Kconfig
> +++ b/arch/arm/kvm/Kconfig
> @@ -24,6 +24,7 @@ config KVM
>  	select HAVE_KVM_ARCH_TLB_FLUSH_ALL
>  	select KVM_MMIO
>  	select KVM_ARM_HOST
> +	select KVM_GENERIC_DIRTYLOG_READ_PROTECT
>  	depends on ARM_VIRT_EXT && ARM_LPAE
>  	---help---
>  	  Support hosting virtualized guest machines. You will also
> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
> index a99e0cd..040c0f3 100644
> --- a/arch/arm/kvm/arm.c
> +++ b/arch/arm/kvm/arm.c
> @@ -737,9 +737,55 @@ long kvm_arch_vcpu_ioctl(struct file *filp,
>  	}
>  }
>  
> +/**
> + * kvm_vm_ioctl_get_dirty_log - get and clear the log of dirty pages in a slot
> + * @kvm:	kvm instance
> + * @log:	slot id and address to which we copy the log
> + *
> + * We need to keep it in mind that VCPU threads can write to the bitmap
> + * concurrently.  So, to avoid losing data, we keep the following order for
> + * each bit:
> + *
> + *   1. Take a snapshot of the bit and clear it if needed.
> + *   2. Write protect the corresponding page.
> + *   3. Copy the snapshot to the userspace.
> + *   4. Flush TLB's if needed.
> + *
> + * Steps 1,2,3 are handled by kvm_get_dirty_log_protect().
> + * Between 2 and 4, the guest may write to the page using the remaining TLB
> + * entry.  This is not a problem because the page is reported dirty using
> + * the snapshot taken before and step 4 ensures that writes done after
> + * exiting to userspace will be logged for the next call.
> + */
>  int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
>  {
> +#ifdef CONFIG_ARM
> +	int r;
> +	bool is_dirty = false;
> +
> +	mutex_lock(&kvm->slots_lock);
> +
> +	r = kvm_get_dirty_log_protect(kvm, log, &is_dirty);
> +	if (r)
> +		goto out;
> +
> +	/*
> +	 * kvm_get_dirty_log_protect() may fail and we may skip TLB flush
> +	 * leaving few stale spte TLB entries which is harmless, given we're
> +	 * just write protecting spte's, so few stale TLB's will be left in
> +	 * original R/W state. And since the bitmap is corrupt userspace will
> +	 * error out anyway (i.e. during migration or dirty page loging for

s/loging/logging/

Hmmm, where is this behavior specified in the ABI?  If you call
KVM_GET_DIRTY_LOG subsequently, you will now potentially have unreported
dirty pages, which can be completely avoided by removing the
if-statement and the goto above.  Why not simply do that and get rid of
this comment?

> +	 * other reasons) terminating dirty page logging.
> +	 */
> +	if (is_dirty)
> +		kvm_flush_remote_tlbs(kvm);
> +out:
> +	mutex_unlock(&kvm->slots_lock);
> +
> +	return r;
> +#else /* ARM64 */
>  	return -EINVAL;
> +#endif
>  }
>  
>  static int kvm_vm_ioctl_set_device_addr(struct kvm *kvm,
> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
> index 1e8b6a9..8137455 100644
> --- a/arch/arm/kvm/mmu.c
> +++ b/arch/arm/kvm/mmu.c
> @@ -870,6 +870,28 @@ void kvm_mmu_wp_memory_region(struct kvm *kvm, int slot)
>  	spin_unlock(&kvm->mmu_lock);
>  	kvm_flush_remote_tlbs(kvm);
>  }
> +
> +/**
> + * kvm_arch_mmu_write_protect_pt_masked() - write protect dirty pages
> + * @kvm:	The KVM pointer
> + * @slot:	The memory slot associated with mask
> + * @gfn_offset:	The gfn offset in memory slot
> + * @mask:	The mask of dirty pages at offset 'gfn_offset' in this memory
> + *              slot to be write protected
> + *
> + * Walks bits set in mask write protects the associated pte's. Caller must
> + * acquire kvm_mmu_lock.
> + */
> +void kvm_arch_mmu_write_protect_pt_masked(struct kvm *kvm,
> +		struct kvm_memory_slot *slot,
> +		gfn_t gfn_offset, unsigned long mask)
> +{
> +	phys_addr_t base_gfn = slot->base_gfn + gfn_offset;
> +	phys_addr_t start = (base_gfn +  __ffs(mask)) << PAGE_SHIFT;
> +	phys_addr_t end = (base_gfn + __fls(mask) + 1) << PAGE_SHIFT;
> +
> +	stage2_wp_range(kvm, start, end);
> +}
>  #endif
>  
>  static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> -- 
> 1.7.9.5
>