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

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] arm64: kvm: UAPI breakage for dirty ring size on 64KB page systems
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commit 38dde0ab2f10c37402c620c7b1328624808205ad
Author: Tian Zheng <zhengtian10@huawei.com>

KVM: arm64: Reserve dirty ring space for the HDBSS buffer

This commit reserves space in the dirty ring for CPU-side dirty buffers by
implementing kvm_cpu_dirty_log_size(). It also pins the HDBSS buffer size to
PAGE_SIZE when the dirty ring is enabled.

> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 7bf82d65041c8..c1e09ba98d48a 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
[ ... ]
> @@ -2841,3 +2841,11 @@ void kvm_toggle_cache(struct kvm_vcpu *vcpu, bool was_enabled)
>  
>  	trace_kvm_toggle_cache(*vcpu_pc(vcpu), was_enabled, now_enabled);
>  }
> +
> +int kvm_cpu_dirty_log_size(struct kvm *kvm)
> +{
> +	if (!system_supports_hdbss())
> +		return 0;
> +
> +	return kvm_hdbss_buffer_size(kvm) / sizeof(u64);
> +}

[Severity: High]
Does this calculation break the UAPI for standard dirty ring size requests on
64KB page systems?

Since KVM_ARM_HDBSS_DEFAULT_SIZE is tied to PAGE_SIZE, this calculation scales
with the system page size. On a 64KB page system, kvm_hdbss_buffer_size() yields
65536 bytes, returning 8192 reserved entries (65536 / 8).

When a userspace process requests a standard dirty ring size of 65536 bytes
(64KB) via KVM_ENABLE_CAP for KVM_CAP_DIRTY_LOG_RING, it seems the ioctl will
now fail:

virt/kvm/kvm_main.c:kvm_vm_ioctl_enable_cap() {
    ...
	if (size < kvm_dirty_ring_get_rsvd_entries(kvm) *
	    sizeof(struct kvm_dirty_gfn) || size < PAGE_SIZE)
		return -EINVAL;
    ...
}

Because (8192 + 64) * 16 = 132096 bytes is greater than the requested 65536
bytes, the ioctl rejects previously valid userspace configurations with -EINVAL,
which would cause QEMU and other VMMs to fail to start VMs.

Could this buffer size be decoupled from PAGE_SIZE to avoid the entry inflation
and ABI breakage on 64KB page systems?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260929103655.85107-1-zhengtian10@huawei.com?part=11