linux-kernel - Re: [PATCH 0/7] KVM: Alleviate mmu_lock hold time when we start dirty logging

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Date:	Thu, 20 Dec 2012 23:35:45 +0900
From:	Takuya Yoshikawa <takuya.yoshikawa@...il.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Gleb Natapov <gleb@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/7] KVM: Alleviate mmu_lock hold time when we start
 dirty logging

On Thu, 20 Dec 2012 06:41:27 -0700
Alex Williamson <alex.williamson@...hat.com> wrote:

> Hmm, isn't the fix as simple as:
> 
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -847,7 +847,8 @@ int __kvm_set_memory_region(struct kvm *kvm,
>                                 GFP_KERNEL);
>                 if (!slots)
>                         goto out_free;
> -       }
> +       } else
> +               slots->generation = kvm->memslots->generation;
>  
>         /* map new memory slot into the iommu */
>         if (npages) {
> 
> Or even just slots->generation++ since we're holding the lock across all
> of this.

Yes, the fix should work, but I do not want to update the
generation from outside of update_memslots().

> The original patch can be reverted, there are no following dependencies,
> but the idea was that we're making the memslot array larger, so there
> could be more pressure in allocating it, so let's not trivially do extra
> frees and allocs.  Thanks,

I agree that the current set_memory_region() is not good for frequent updates.
But the alloc/free is not a major factor at the moment: flushing shadows should
be more problematic.

Thanks,
	Takuya
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