Re: [PATCH v5 5/7] KVM: PPC: Book3S HV: migrate hot plugged memory

[Bharata B Rao <bharata@linux.ibm.com>]

On Thu, Jul 23, 2020 at 01:07:22PM -0700, Ram Pai wrote:
> From: Laurent Dufour <ldufour@linux.ibm.com>
> 
> When a memory slot is hot plugged to a SVM, PFNs associated with the
> GFNs in that slot must be migrated to the secure-PFNs, aka device-PFNs.
> 
> Call kvmppc_uv_migrate_mem_slot() to accomplish this.
> Disable page-merge for all pages in the memory slot.
> 
> Signed-off-by: Ram Pai <linuxram@us.ibm.com>
> [rearranged the code, and modified the commit log]
> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> ---
>  arch/powerpc/include/asm/kvm_book3s_uvmem.h | 14 ++++++++++++++
>  arch/powerpc/kvm/book3s_hv.c                | 10 ++--------
>  arch/powerpc/kvm/book3s_hv_uvmem.c          | 23 +++++++++++++++++++----
>  3 files changed, 35 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/kvm_book3s_uvmem.h b/arch/powerpc/include/asm/kvm_book3s_uvmem.h
> index f229ab5..59c17ca 100644
> --- a/arch/powerpc/include/asm/kvm_book3s_uvmem.h
> +++ b/arch/powerpc/include/asm/kvm_book3s_uvmem.h
> @@ -25,6 +25,10 @@ void kvmppc_uvmem_drop_pages(const struct kvm_memory_slot *free,
>  			     struct kvm *kvm, bool skip_page_out);
>  int kvmppc_uv_migrate_mem_slot(struct kvm *kvm,
>  			const struct kvm_memory_slot *memslot);
> +int kvmppc_uvmem_memslot_create(struct kvm *kvm,
> +		const struct kvm_memory_slot *new);
> +void kvmppc_uvmem_memslot_delete(struct kvm *kvm,
> +		const struct kvm_memory_slot *old);
>  #else
>  static inline int kvmppc_uvmem_init(void)
>  {
> @@ -84,5 +88,15 @@ static inline int kvmppc_send_page_to_uv(struct kvm *kvm, unsigned long gfn)
>  static inline void
>  kvmppc_uvmem_drop_pages(const struct kvm_memory_slot *free,
>  			struct kvm *kvm, bool skip_page_out) { }
> +
> +static inline int  kvmppc_uvmem_memslot_create(struct kvm *kvm,
> +		const struct kvm_memory_slot *new)
> +{
> +	return H_UNSUPPORTED;
> +}
> +
> +static inline void  kvmppc_uvmem_memslot_delete(struct kvm *kvm,
> +		const struct kvm_memory_slot *old) { }
> +
>  #endif /* CONFIG_PPC_UV */
>  #endif /* __ASM_KVM_BOOK3S_UVMEM_H__ */
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index d331b46..b1485ca 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -4515,16 +4515,10 @@ static void kvmppc_core_commit_memory_region_hv(struct kvm *kvm,
>  
>  	switch (change) {
>  	case KVM_MR_CREATE:
> -		if (kvmppc_uvmem_slot_init(kvm, new))
> -			return;
> -		uv_register_mem_slot(kvm->arch.lpid,
> -				     new->base_gfn << PAGE_SHIFT,
> -				     new->npages * PAGE_SIZE,
> -				     0, new->id);
> +		kvmppc_uvmem_memslot_create(kvm, new);

Only concern is that kvmppc_uvmem_memslot_create() can fail due
to multiple reasons but we ignore them and go ahead with memory
hotplug.

May be this hasn't been observed in reality but if we can note this
as a TODO in the comments to dig further and explore the possibility
of recovering from here, then

Reviewed-by: Bharata B Rao <bharata@linux.ibm.com>

Regards,
Bharata.