linux-kernel - Re: [PATCH v3 kvm/queue 06/16] KVM: Implement fd-based memory using MEMFD

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Date:   Thu, 23 Dec 2021 18:34:22 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Chao Peng <chao.p.peng@...ux.intel.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        qemu-devel@...gnu.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, john.ji@...el.com, susie.li@...el.com,
        jun.nakajima@...el.com, dave.hansen@...el.com, ak@...ux.intel.com,
        david@...hat.com
Subject: Re: [PATCH v3 kvm/queue 06/16] KVM: Implement fd-based memory using
 MEMFD_OPS interfaces

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
>