Re: [PATCH v5 11/15] KVM: arm64: Reserve dirty ring space for the HDBSS buffer

[Tian Zheng <zhengtian10@huawei.com>]

On 11/2/2026 10:54 PM, Leonardo Bras wrote:
> On Tue, Sep 29, 2026 at 06:36:51PM +0800, Tian Zheng wrote:
>> The dirty ring reserves room for CPU-side dirty buffers via
>> kvm_cpu_dirty_log_size(). Report the HDBSS buffer entry count
>> through it, so a full buffer flush always fits in the ring.
>>
>> Pin the HDBSS buffer size to PAGE_SIZE when dirty-ring is enabled,
>> via kvm_arch_dirty_ring_size_updated(): a larger buffer only
>> shrinks the soft_limit and forces more frequent userspace drains.
>>
>> Signed-off-by: Tian Zheng <zhengtian10@huawei.com>
>> ---
>>   arch/arm64/kvm/arm.c | 8 ++++++++
>>   arch/arm64/kvm/mmu.c | 8 ++++++++
>>   2 files changed, 16 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index 9d7bdece149a..5a7046acb62e 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -215,6 +215,14 @@ static int kvm_arm_default_max_vcpus(void)
>>   	return vgic_present ? kvm_vgic_get_max_vcpus() : KVM_MAX_VCPUS;
>>   }
>>
>> +void kvm_arch_dirty_ring_size_updated(struct kvm *kvm)
>> +{
>> +	if (!system_supports_hdbss())
>> +		return;
>> +
>> +	kvm->arch.hdbss_buffer_size = KVM_ARM_HDBSS_DEFAULT_SIZE;
>> +}
>> +
> 
> On the same subject as a previous patch:
> I don't think this is the best approach, on top of adding a new weak
> function to take care.
> 
> Maybe during the dirty-ring allocation, on vcpu creation, we could take
> into account the requested dirty-ring size, and just allocate the HDBSS
> buffer taking that into account. That would make it simpler, don't you
> agree?
> 
> Thanks!
> Leo

Hi Leo,

Same answer as I explained in [1]. Anyway, I'll remove this function.

[1] 
https://lore.kernel.org/all/afcbf19b-bd13-4afc-ab56-8bc50c793e50@huawei.com/

Thanks,
Tian