Re: [PATCH v5 12/13] KVM: Expose KVM_MEM_PRIVATE

[Chao Peng]

On Tue, Mar 29, 2022 at 07:13:00PM +0000, Sean Christopherson wrote:
> On Thu, Mar 10, 2022, Chao Peng wrote:
> > KVM_MEM_PRIVATE is not exposed by default but architecture code can turn
> > on it by implementing kvm_arch_private_memory_supported().
> > 
> > Signed-off-by: Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > Signed-off-by: Chao Peng <chao.p.peng@xxxxxxxxxxxxxxx>
> > ---
> >  include/linux/kvm_host.h |  1 +
> >  virt/kvm/kvm_main.c      | 24 +++++++++++++++++++-----
> >  2 files changed, 20 insertions(+), 5 deletions(-)
> > 
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index 186b9b981a65..0150e952a131 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1432,6 +1432,7 @@ bool kvm_arch_dy_has_pending_interrupt(struct kvm_vcpu *vcpu);
> >  int kvm_arch_post_init_vm(struct kvm *kvm);
> >  void kvm_arch_pre_destroy_vm(struct kvm *kvm);
> >  int kvm_arch_create_vm_debugfs(struct kvm *kvm);
> > +bool kvm_arch_private_memory_supported(struct kvm *kvm);
> >  
> >  #ifndef __KVM_HAVE_ARCH_VM_ALLOC
> >  /*
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 52319f49d58a..df5311755a40 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1485,10 +1485,19 @@ static void kvm_replace_memslot(struct kvm *kvm,
> >  	}
> >  }
> >  
> > -static int check_memory_region_flags(const struct kvm_userspace_memory_region *mem)
> > +bool __weak kvm_arch_private_memory_supported(struct kvm *kvm)
> > +{
> > +	return false;
> > +}
> > +
> > +static int check_memory_region_flags(struct kvm *kvm,
> > +				const struct kvm_userspace_memory_region *mem)
> >  {
> >  	u32 valid_flags = KVM_MEM_LOG_DIRTY_PAGES;
> >  
> > +	if (kvm_arch_private_memory_supported(kvm))
> > +		valid_flags |= KVM_MEM_PRIVATE;
> > +
> >  #ifdef __KVM_HAVE_READONLY_MEM
> >  	valid_flags |= KVM_MEM_READONLY;
> >  #endif
> > @@ -1900,7 +1909,7 @@ int __kvm_set_memory_region(struct kvm *kvm,
> >  	int as_id, id;
> >  	int r;
> >  
> > -	r = check_memory_region_flags(mem);
> > +	r = check_memory_region_flags(kvm, mem);
> >  	if (r)
> >  		return r;
> >  
> > @@ -1913,10 +1922,12 @@ int __kvm_set_memory_region(struct kvm *kvm,
> >  		return -EINVAL;
> >  	if (mem->guest_phys_addr & (PAGE_SIZE - 1))
> >  		return -EINVAL;
> > -	/* We can read the guest memory with __xxx_user() later on. */
> >  	if ((mem->userspace_addr & (PAGE_SIZE - 1)) ||
> > -	    (mem->userspace_addr != untagged_addr(mem->userspace_addr)) ||
> > -	     !access_ok((void __user *)(unsigned long)mem->userspace_addr,
> > +	    (mem->userspace_addr != untagged_addr(mem->userspace_addr)))
> > +		return -EINVAL;
> > +	/* We can read the guest memory with __xxx_user() later on. */
> > +	if (!(mem->flags & KVM_MEM_PRIVATE) &&
> > +	    !access_ok((void __user *)(unsigned long)mem->userspace_addr,
> 
> This should sanity check private_offset for private memslots.  At a bare minimum,
> wrapping should be disallowed.

Will add this.

> 
> >  			mem->memory_size))
> >  		return -EINVAL;
> >  	if (as_id >= KVM_ADDRESS_SPACE_NUM || id >= KVM_MEM_SLOTS_NUM)
> > @@ -1957,6 +1968,9 @@ int __kvm_set_memory_region(struct kvm *kvm,
> >  		if ((kvm->nr_memslot_pages + npages) < kvm->nr_memslot_pages)
> >  			return -EINVAL;
> >  	} else { /* Modify an existing slot. */
> > +		/* Private memslots are immutable, they can only be deleted. */
> > +		if (mem->flags & KVM_MEM_PRIVATE)
> > +			return -EINVAL;
> 
> These sanity checks belong in "KVM: Register private memslot to memory backing store",
> e.g. that patch is "broken" without the immutability restriction.  It's somewhat moot
> because the code is unreachable, but it makes reviewing confusing/difficult.
> 
> But rather than move the sanity checks back, I think I'd prefer to pull all of patch 10
> here.  I think it also makes sense to drop "KVM: Use memfile_pfn_ops to obtain pfn for
> private pages" and add the pointer in "struct kvm_memory_slot" in patch "KVM: Extend the
> memslot to support fd-based private memory", with the use of the ops folded into
> "KVM: Handle page fault for private memory".  Adding code to KVM and KVM-x86 in a single
> patch is ok, and overall makes things easier to review because the new helpers have a
> user right away, especially since there will be #ifdeffery.
> 
> I.e. end up with something like:
> 
>   mm: Introduce memfile_notifier
>   mm/shmem: Restrict MFD_INACCESSIBLE memory against RLIMIT_MEMLOCK
>   KVM: Extend the memslot to support fd-based private memory
>   KVM: Use kvm_userspace_memory_region_ext
>   KVM: Add KVM_EXIT_MEMORY_ERROR exit
>   KVM: Handle page fault for private memory
>   KVM: Register private memslot to memory backing store
>   KVM: Zap existing KVM mappings when pages changed in the private fd
>   KVM: Enable and expose KVM_MEM_PRIVATE

Thanks for the suggestion. That makes sense.

Chao
> 
> >  		if ((mem->userspace_addr != old->userspace_addr) ||
> >  		    (npages != old->npages) ||
> >  		    ((mem->flags ^ old->flags) & KVM_MEM_READONLY))
> > -- 
> > 2.17.1
> >