Re: [PATCH v3 kvm/queue 06/16] KVM: Implement fd-based memory using MEMFD_OPS interfaces

[Sean Christopherson <seanjc@google.com>]

On Thu, Dec 23, 2021, Chao Peng wrote:
> diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
> index 03b2ce34e7f4..86655cd660ca 100644
> --- a/arch/x86/kvm/Kconfig
> +++ b/arch/x86/kvm/Kconfig
> @@ -46,6 +46,7 @@ config KVM
>  	select SRCU
>  	select INTERVAL_TREE
>  	select HAVE_KVM_PM_NOTIFIER if PM
> +	select MEMFD_OPS

MEMFD_OPS is a weird Kconfig name given that it's not just memfd() that can
implement the ops.

>  	help
>  	  Support hosting fully virtualized guest machines using hardware
>  	  virtualization extensions.  You will need a fairly recent
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 3bd875f9669f..21f8b1880723 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -806,6 +806,12 @@ static inline void kvm_irqfd_exit(void)
>  {
>  }
>  #endif
> +
> +int kvm_memfd_register(struct kvm *kvm, struct kvm_memory_slot *slot);
> +void kvm_memfd_unregister(struct kvm_memory_slot *slot);
> +long kvm_memfd_get_pfn(struct kvm_memory_slot *slot, gfn_t gfn, int *order);
> +void kvm_memfd_put_pfn(kvm_pfn_t pfn);
> +
>  int kvm_init(void *opaque, unsigned vcpu_size, unsigned vcpu_align,
>  		  struct module *module);
>  void kvm_exit(void);
> diff --git a/virt/kvm/Makefile.kvm b/virt/kvm/Makefile.kvm
> index ffdcad3cc97a..8842128d8429 100644
> --- a/virt/kvm/Makefile.kvm
> +++ b/virt/kvm/Makefile.kvm
> @@ -5,7 +5,7 @@
>  
>  KVM ?= ../../../virt/kvm
>  
> -kvm-y := $(KVM)/kvm_main.o $(KVM)/eventfd.o $(KVM)/binary_stats.o
> +kvm-y := $(KVM)/kvm_main.o $(KVM)/eventfd.o $(KVM)/binary_stats.o $(KVM)/memfd.o

This should be

   kvm-$(CONFIG_MEMFD_OPS) += $(KVM)/memfd.o

with stubs provided in a header file as needed.  I also really dislike naming KVM's
file memfd.c, though I don't have a good alternative off the top of my head.

>  kvm-$(CONFIG_KVM_VFIO) += $(KVM)/vfio.o
>  kvm-$(CONFIG_KVM_MMIO) += $(KVM)/coalesced_mmio.o
>  kvm-$(CONFIG_KVM_ASYNC_PF) += $(KVM)/async_pf.o


> +#ifdef CONFIG_MEMFD_OPS
> +static const struct memfd_pfn_ops *memfd_ops;

memfd_ops needs to be associated with the slot, e.g. userspace should be able to
map multiple types of a backing stores into a single VM.  This doesn't even allow
that for multiple VMs, and there are all kinds of ordering issues.

> +void kvm_memfd_unregister(struct kvm_memory_slot *slot)
> +{
> +#ifdef CONFIG_MEMFD_OPS
> +	if (slot->file) {
> +		fput(slot->file);

Needs to actually unregister.

> +		slot->file = NULL;
> +	}
> +#endif
> +}
> -- 
> 2.17.1
>