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

[Tian Zheng]

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@xxxxxxxxxx>
> > ---
> >   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@xxxxxxxxxx/

Thanks,
Tian