Re: [PATCH v1 1/5] KVM: arm64: Enable ring-based dirty memory tracking

From: Marc Zyngier <maz@kernel.org>
To: Gavin Shan <gshan@redhat.com>
Cc: kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
	linux-kselftest@vger.kernel.org, peterx@redhat.com,
	pbonzini@redhat.com, corbet@lwn.net, james.morse@arm.com,
	alexandru.elisei@arm.com, suzuki.poulose@arm.com,
	oliver.upton@linux.dev, catalin.marinas@arm.com, will@kernel.org,
	shuah@kernel.org, seanjc@google.com, drjones@redhat.com,
	dmatlack@google.com, bgardon@google.com, ricarkol@google.com,
	zhenyzha@redhat.com, shan.gavin@gmail.com
Subject: Re: [PATCH v1 1/5] KVM: arm64: Enable ring-based dirty memory tracking
Date: Fri, 19 Aug 2022 09:00:18 +0100	[thread overview]
Message-ID: <87lerkwtm5.wl-maz@kernel.org> (raw)
In-Reply-To: <20220819005601.198436-2-gshan@redhat.com>

On Fri, 19 Aug 2022 01:55:57 +0100,
Gavin Shan <gshan@redhat.com> wrote:
> 
> The ring-based dirty memory tracking has been available and enabled
> on x86 for a while. The feature is beneficial when the number of
> dirty pages is small in a checkpointing system or live migration
> scenario. More details can be found from fb04a1eddb1a ("KVM: X86:
> Implement ring-based dirty memory tracking").
> 
> This enables the ring-based dirty memory tracking on ARM64. It's
> notable that no extra reserved ring entries are needed on ARM64
> because the huge pages are always split into base pages when page
> dirty tracking is enabled.

Can you please elaborate on this? Adding a per-CPU ring of course
results in extra memory allocation, so there must be a subtle
x86-specific detail that I'm not aware of...

> 
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>  Documentation/virt/kvm/api.rst    | 2 +-
>  arch/arm64/include/uapi/asm/kvm.h | 1 +
>  arch/arm64/kvm/Kconfig            | 1 +
>  arch/arm64/kvm/arm.c              | 8 ++++++++
>  4 files changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index abd7c32126ce..19fa1ac017ed 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -8022,7 +8022,7 @@ regardless of what has actually been exposed through the CPUID leaf.
>  8.29 KVM_CAP_DIRTY_LOG_RING
>  ---------------------------
>  
> -:Architectures: x86
> +:Architectures: x86, arm64
>  :Parameters: args[0] - size of the dirty log ring
>  
>  KVM is capable of tracking dirty memory using ring buffers that are
> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
> index 3bb134355874..7e04b0b8d2b2 100644
> --- a/arch/arm64/include/uapi/asm/kvm.h
> +++ b/arch/arm64/include/uapi/asm/kvm.h
> @@ -43,6 +43,7 @@
>  #define __KVM_HAVE_VCPU_EVENTS
>  
>  #define KVM_COALESCED_MMIO_PAGE_OFFSET 1
> +#define KVM_DIRTY_LOG_PAGE_OFFSET 64

For context, the documentation says:

<quote>
- if KVM_CAP_DIRTY_LOG_RING is available, a number of pages at
  KVM_DIRTY_LOG_PAGE_OFFSET * PAGE_SIZE. [...]
</quote>

What is the reason for picking this particular value?

>  
>  #define KVM_REG_SIZE(id)						\
>  	(1U << (((id) & KVM_REG_SIZE_MASK) >> KVM_REG_SIZE_SHIFT))
> diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> index 815cc118c675..0309b2d0f2da 100644
> --- a/arch/arm64/kvm/Kconfig
> +++ b/arch/arm64/kvm/Kconfig
> @@ -32,6 +32,7 @@ menuconfig KVM
>  	select KVM_VFIO
>  	select HAVE_KVM_EVENTFD
>  	select HAVE_KVM_IRQFD
> +	select HAVE_KVM_DIRTY_RING
>  	select HAVE_KVM_MSI
>  	select HAVE_KVM_IRQCHIP
>  	select HAVE_KVM_IRQ_ROUTING
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 986cee6fbc7f..3de6b9b39db7 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -866,6 +866,14 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>  		if (!ret)
>  			ret = 1;
>  
> +		/* Force vcpu exit if its dirty ring is soft-full */
> +		if (unlikely(vcpu->kvm->dirty_ring_size &&
> +			     kvm_dirty_ring_soft_full(&vcpu->dirty_ring))) {
> +			vcpu->run->exit_reason = KVM_EXIT_DIRTY_RING_FULL;
> +			trace_kvm_dirty_ring_exit(vcpu);
> +			ret = 0;
> +		}
> +

Why can't this be moved to kvm_vcpu_exit_request() instead? I would
also very much like the check to be made a common helper with x86.

A seemingly approach would be to make this a request on dirty log
insertion, and avoid the whole "check the log size" on every run,
which adds pointless overhead to unsuspecting users (aka everyone).

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.