Re: [PATCH v4] KVM: x86: enable dirty log gradually in small chunks

[Peter Xu <peterx@redhat.com>]

On Thu, Feb 27, 2020 at 09:32:27AM +0800, Jay Zhou wrote:
> It could take kvm->mmu_lock for an extended period of time when
> enabling dirty log for the first time. The main cost is to clear
> all the D-bits of last level SPTEs. This situation can benefit from
> manual dirty log protect as well, which can reduce the mmu_lock
> time taken. The sequence is like this:
> 
> 1. Initialize all the bits of the dirty bitmap to 1 when enabling
>    dirty log for the first time
> 2. Only write protect the huge pages
> 3. KVM_GET_DIRTY_LOG returns the dirty bitmap info
> 4. KVM_CLEAR_DIRTY_LOG will clear D-bit for each of the leaf level
>    SPTEs gradually in small chunks
> 
> Under the Intel(R) Xeon(R) Gold 6152 CPU @ 2.10GHz environment,
> I did some tests with a 128G windows VM and counted the time taken
> of memory_global_dirty_log_start, here is the numbers:
> 
> VM Size        Before    After optimization
> 128G           460ms     10ms
> 
> Signed-off-by: Jay Zhou <jianjay.zhou@huawei.com>

Reviewed-by: Peter Xu <peterx@redhat.com>

-- 
Peter Xu