Re: [PATCH] KVM: Remove duplicated zero clear with dirty_bitmap buffer

From: maobibo
Date: Thu Jun 13 2024 - 22:46:32 EST




On 2024/6/14 上午3:25, Christophe JAILLET wrote:
Le 13/06/2024 à 14:54, Bibo Mao a écrit :
Since dirty_bitmap pointer is allocated with function __vcalloc(),
there is __GFP_ZERO flag set in the implementation about this function
__vcalloc_noprof(). It is not necessary to clear dirty_bitmap buffer
with zero again.

Signed-off-by: Bibo Mao <maobibo@xxxxxxxxxxx>
---
  virt/kvm/kvm_main.c | 3 ---
  1 file changed, 3 deletions(-)


Hi,

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 14841acb8b95..c7d4a041dcfa 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1669,9 +1669,6 @@ static int kvm_prepare_memory_region(struct kvm *kvm,
              r = kvm_alloc_dirty_bitmap(new);
              if (r)
                  return r;
-
-            if (kvm_dirty_log_manual_protect_and_init_set(kvm))
-                bitmap_set(new->dirty_bitmap, 0, new->npages);

unless I miss something obvious, this does not clear anything, but set all bits to 1.

0 is not for "write 0" (i.e. clear), but for "start at offset 0".
So all bits are set to 1 in this case.
you are right, I had thought it is to write 0 :(

I do not know whether KVM_DIRTY_LOG_INITIALLY_SET should be enabled on LoongArch. If it is set, write protection for second MMU will start one by one in function kvm_arch_mmu_enable_log_dirty_pt_masked() when dirty log is cleared if it is set, else write protection will start in function kvm_arch_commit_memory_region() when flag of memslot is changed.

I do not see the obvious benefits between these two write protect stages. Can anyone give me any hints?

Regards
Bibo Mao


CJ

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base-commit: 83a7eefedc9b56fe7bfeff13b6c7356688ffa670