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

[Leonardo Bras <leo.bras@arm.com>]

On Fri, Nov 06, 2026 at 06:27:27PM +0800, Tian Zheng wrote:
> 
> 
> 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.
> 

Awesome, thanks!
Leo