2024-06-13 12:30:45

by Bibo Mao

Subject: [PATCH] KVM: Discard zero mask with function kvm_dirty_ring_reset

Function kvm_reset_dirty_gfn may be called with parameters cur_slot /
cur_offset / mask are all zero, it does not represent real dirty page.
It is not necessary to clear dirty page in this condition. Also return
value of macro __fls() is undefined if mask is zero which is called in
funciton kvm_reset_dirty_gfn(). Here just discard it.

Signed-off-by: Bibo Mao <[email protected]>
---
virt/kvm/dirty_ring.c | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index 86d267db87bb..05f4c1c40cc7 100644
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -147,14 +147,16 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
continue;
}
}
- kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
+ if (mask)
+ kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
cur_slot = next_slot;
cur_offset = next_offset;
mask = 1;
first_round = false;
}

- kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
+ if (mask)
+ kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);

/*
* The request KVM_REQ_DIRTY_RING_SOFT_FULL will be cleared

base-commit: 83a7eefedc9b56fe7bfeff13b6c7356688ffa670
--
2.39.3

2024-06-13 16:44:03

by Sean Christopherson

Subject: Re: [PATCH] KVM: Discard zero mask with function kvm_dirty_ring_reset

On Thu, Jun 13, 2024, Bibo Mao wrote:
> Function kvm_reset_dirty_gfn may be called with parameters cur_slot /
> cur_offset / mask are all zero, it does not represent real dirty page.
> It is not necessary to clear dirty page in this condition. Also return
> value of macro __fls() is undefined if mask is zero which is called in
> funciton kvm_reset_dirty_gfn(). Here just discard it.
>
> Signed-off-by: Bibo Mao <[email protected]>
> ---
> virt/kvm/dirty_ring.c | 6 ++++--
> 1 file changed, 4 insertions(+), 2 deletions(-)
>
> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> index 86d267db87bb..05f4c1c40cc7 100644
> --- a/virt/kvm/dirty_ring.c
> +++ b/virt/kvm/dirty_ring.c
> @@ -147,14 +147,16 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
> continue;
> }
> }
> - kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> + if (mask)
> + kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> cur_slot = next_slot;
> cur_offset = next_offset;
> mask = 1;
> first_round = false;
> }
>
> - kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
> + if (mask)
> + kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);

Given that mask must be checked before __fls(), just do:

diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
index 86d267db87bb..7bc74969a819 100644
--- a/virt/kvm/dirty_ring.c
+++ b/virt/kvm/dirty_ring.c
@@ -55,6 +55,9 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
struct kvm_memory_slot *memslot;
int as_id, id;

+ if (!mask)
+ return;
+
as_id = slot >> 16;
id = (u16)slot;

2024-06-14 02:58:33

by Bibo Mao

Subject: Re: [PATCH] KVM: Discard zero mask with function kvm_dirty_ring_reset

On 2024/6/14 上午12:43, Sean Christopherson wrote:
> On Thu, Jun 13, 2024, Bibo Mao wrote:
>> Function kvm_reset_dirty_gfn may be called with parameters cur_slot /
>> cur_offset / mask are all zero, it does not represent real dirty page.
>> It is not necessary to clear dirty page in this condition. Also return
>> value of macro __fls() is undefined if mask is zero which is called in
>> funciton kvm_reset_dirty_gfn(). Here just discard it.
>>
>> Signed-off-by: Bibo Mao <[email protected]>
>> ---
>> virt/kvm/dirty_ring.c | 6 ++++--
>> 1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
>> index 86d267db87bb..05f4c1c40cc7 100644
>> --- a/virt/kvm/dirty_ring.c
>> +++ b/virt/kvm/dirty_ring.c
>> @@ -147,14 +147,16 @@ int kvm_dirty_ring_reset(struct kvm *kvm, struct kvm_dirty_ring *ring)
>> continue;
>> }
>> }
>> - kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
>> + if (mask)
>> + kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
>> cur_slot = next_slot;
>> cur_offset = next_offset;
>> mask = 1;
>> first_round = false;
>> }
>>
>> - kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
>> + if (mask)
>> + kvm_reset_dirty_gfn(kvm, cur_slot, cur_offset, mask);
>
> Given that mask must be checked before __fls(), just do:
That is ok for me. To be frankly I am not familiar with kvm common code,
I submit this patch just when I look through the migration source code.

Regards
Bibo Mao
>
> diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> index 86d267db87bb..7bc74969a819 100644
> --- a/virt/kvm/dirty_ring.c
> +++ b/virt/kvm/dirty_ring.c
> @@ -55,6 +55,9 @@ static void kvm_reset_dirty_gfn(struct kvm *kvm, u32 slot, u64 offset, u64 mask)
> struct kvm_memory_slot *memslot;
> int as_id, id;
>
> + if (!mask)
> + return;
> +
> as_id = slot >> 16;
> id = (u16)slot;
>