Re: [PATCH 5/7] KVM: arm64: Add memory_slot_unmaped counter for kvm_stat

[Marc Zyngier <maz@kernel.org>]

On Fri, 19 Mar 2021 16:17:09 +0000,
Yoan Picchi <yoan.picchi@arm.com> wrote:
> 
> Add a counter for when a memory slot is unmapped, meaning that all
> memory belonging to a specific VM is made available to be mapped for
> anoother VM.

No. It just means it is unmapped from the guest. The VMM still has the
pages.

> A memory slot is the memory that can be used to hold a
> guest's pages and can be made available by Qemu for instance. For
> now the memory slot are only unmapped when a vm restarts.
> 
> Signed-off-by: Yoan Picchi <yoan.picchi@arm.com>
> ---
>  arch/arm64/include/asm/kvm_host.h | 1 +
>  arch/arm64/kvm/guest.c            | 1 +
>  arch/arm64/kvm/mmu.c              | 1 +
>  3 files changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index e6967951f..76c81aa79 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -548,6 +548,7 @@ static inline bool __vcpu_write_sys_reg_to_cpu(u64 val, int reg)
>  struct kvm_vm_stat {
>  	ulong remote_tlb_flush;
>  	ulong flush_all_cache_lines;
> +	ulong memory_slot_unmaped;
>  	ulong cached_page_invalidated;
>  };
>  
> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
> index d948c3bd2..cd227136e 100644
> --- a/arch/arm64/kvm/guest.c
> +++ b/arch/arm64/kvm/guest.c
> @@ -42,6 +42,7 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
>  	VCPU_STAT("huge_page_mapped", huge_page_mapped),
>  	VM_STAT("remote_tlb_flush", remote_tlb_flush),
>  	VM_STAT("flush_all_cache_lines", flush_all_cache_lines),
> +	VM_STAT("memory_slot_unmaped", memory_slot_unmaped),
>  	VM_STAT("cached_page_invalidated", cached_page_invalidated),
>  	VCPU_STAT("exits", exits),
>  	VCPU_STAT("halt_poll_success_ns", halt_poll_success_ns),
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 985b048df..1e8aeafc2 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -419,6 +419,7 @@ static void stage2_unmap_memslot(struct kvm *kvm,
>  	phys_addr_t addr = memslot->base_gfn << PAGE_SHIFT;
>  	phys_addr_t size = PAGE_SIZE * memslot->npages;
>  	hva_t reg_end = hva + size;
> +	kvm->stat.memory_slot_unmaped++;

Why is that a relevant thing to track? You even increment the counter
before even checking whether this will unmap anything... :-(

	M.

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