Re: [PATCH 4/4] KVM: Optimize dirty logging by rmap_write_protect()

[Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>]

(2011/11/14 19:22), Avi Kivity wrote:

>> + *
>> + * Generally speaking, if there are not so many dirty pages compared to the
>> + * number of shadow pages, we should use the latter.
>> + *
>> + * Note that letting others write into a page marked dirty in the old bitmap
>> + * by using the remaining tlb entry is not a problem.  That page will become
>> + * write protected again when we flush the tlb and then be reported dirty to
>> + * the user space by copying the old bitmap.
>> + */
>> +static void write_protect_slot(struct kvm *kvm,
>> +			       struct kvm_memory_slot *memslot,
>> +			       unsigned long *dirty_bitmap,
>> +			       unsigned long nr_dirty_pages)
>> +{
>> +	/* Not many dirty pages compared to # of shadow pages. */
>> +	if (nr_dirty_pages<  kvm->arch.n_used_mmu_pages) {
>
> Seems a reasonable heuristic.  In particular, this is always true for
> vga, yes?  That will get the code exercised.

Almost always yes, once the guest warms up and shadow pages are allocated.

	Takuya

>
>> +		unsigned long gfn_offset;
>> +
>> +		for_each_set_bit(gfn_offset, dirty_bitmap, memslot->npages) {
>> +			unsigned long gfn = memslot->base_gfn + gfn_offset;
>> +
>> +			spin_lock(&kvm->mmu_lock);
>> +			kvm_mmu_rmap_write_protect(kvm, gfn, memslot);
>> +			spin_unlock(&kvm->mmu_lock);
>> +		}
>> +		kvm_flush_remote_tlbs(kvm);
>> +	} else {
>> +		spin_lock(&kvm->mmu_lock);
>> +		kvm_mmu_slot_remove_write_access(kvm, memslot->id);
>> +		spin_unlock(&kvm->mmu_lock);
>> +	}
>> +}
>> +
>>
>