Re: [PATCH v5 13/15] KVM: arm64: Add HDBSS buffer size ioctl for dirty-bitmap mode

[Tian Zheng <zhengtian10@huawei.com>]

On 11/2/2026 11:12 PM, Leonardo Bras wrote:
> On Tue, Sep 29, 2026 at 06:36:53PM +0800, Tian Zheng wrote:
>> In dirty-bitmap mode, a larger HDBSS buffer lets the guest dirty more
>> pages between VM exits, at the cost of memory. The default of one page
>> per vCPU is a reasonable starting point, so allow userspace to opt
>> into a larger buffer via KVM_CAP_ARM_HDBSS_BUFFER_SIZE.
>>
>> The size is specified in bytes through KVM_ENABLE_CAP and must be a
>> power of two in [PAGE_SIZE, SZ_2M]: the upper bound is the largest
>> HDBSSBR_EL2.SZ encoding, and the power-of-two requirement matches the
>> buddy allocator. It must be set before any vCPU is created: -EINVAL
>> after vCPUs exist, -EBUSY if already set.
>>
>> The capability is rejected in dirty-ring mode, where the size is
>> pinned by kvm_arch_dirty_ring_size_updated().
>>
>> KVM_CHECK_EXTENSION returns the configured size for a VM, or SZ_2M
>> when queried globally with a NULL kvm.
>>
>> Signed-off-by: Tian Zheng <zhengtian10@huawei.com>
>> ---
>>   arch/arm64/kvm/arm.c     | 34 ++++++++++++++++++++++++++++++++++
>>   include/uapi/linux/kvm.h |  1 +
>>   2 files changed, 35 insertions(+)
>>
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index 5a7046acb62e..eadbf3a88d31 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -19,6 +19,7 @@
>>   #include <linux/kvm.h>
>>   #include <linux/kvm_irqfd.h>
>>   #include <linux/irqbypass.h>
>> +#include <linux/log2.h>
>>   #include <linux/sched/stat.h>
>>   #include <linux/psci.h>
>>   #include <trace/events/kvm.h>
>> @@ -203,6 +204,31 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>>   		r = 0;
>>   		set_bit(KVM_ARCH_FLAG_EXIT_SEA, &kvm->arch.flags);
>>   		break;
>> +	case KVM_CAP_ARM_HDBSS_BUFFER_SIZE: {
>> +		u64 size = cap->args[0];
>> +
>> +		if (!system_supports_hdbss())
>> +			break;
>> +		if (kvm->dirty_ring_size)
>> +			break;
>> +		if (size < KVM_ARM_HDBSS_DEFAULT_SIZE ||
>> +		    size > KVM_ARM_HDBSS_MAX_SIZE)
>> +			break;
>> +		if (!is_power_of_2(size))
>> +			break;
>> +
>> +		mutex_lock(&kvm->lock);
>> +		if (kvm->created_vcpus) {
>> +			r = -EINVAL;
>> +		} else if (kvm->arch.hdbss_buffer_size) {
>> +			r = -EBUSY;
>> +		} else {
>> +			kvm->arch.hdbss_buffer_size = size;
>> +			r = 0;
>> +		}
>> +		mutex_unlock(&kvm->lock);
>> +		break;
>> +	}
>>   	default:
>>   		break;
>>   	}
>> @@ -500,6 +526,14 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>>   		else
>>   			r = KVM_ARM_EAGER_SPLIT_CHUNK_SIZE_DEFAULT;
>>   		break;
>> +	case KVM_CAP_ARM_HDBSS_BUFFER_SIZE:
>> +		if (!system_supports_hdbss())
>> +			r = 0;
>> +		else if (kvm)
>> +			r = kvm->arch.hdbss_buffer_size ?: KVM_ARM_HDBSS_DEFAULT_SIZE;
> 
> This whole thing of allowing hdbss_buf_size to be set only once makes
> everything more complex. Why do we have this limitation again?
> 
> I mean, if we want to restrict size changes after the first hdbss buffer
> gets allocated, it makes sense, other than that, I would like to understand
> the reason for that.
> 
> Thanks!
> Leo

Hi Leo,

You're right, I re-examined the whole flow and the "set once"
restriction isn't actually needed. I'll remove it.

Thanks,
Tian