Re: [PATCH v3 kvm/queue 14/16] KVM: Handle page fault for private memory

[Sean Christopherson <seanjc@google.com>]

On Wed, Jan 05, 2022, Yan Zhao wrote:
> Sorry, maybe I didn't express it clearly.
> 
> As in the kvm_faultin_pfn_private(), 
> static bool kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
> 				    struct kvm_page_fault *fault,
> 				    bool *is_private_pfn, int *r)
> {
> 	int order;
> 	int mem_convert_type;
> 	struct kvm_memory_slot *slot = fault->slot;
> 	long pfn = kvm_memfd_get_pfn(slot, fault->gfn, &order);
> 	...
> }
> Currently, kvm_memfd_get_pfn() is called unconditionally.
> However, if the backend of a private memslot is not memfd, and is device
> fd for example, a different xxx_get_pfn() is required here.

Ya, I've complained about this in a different thread[*].  This should really be
something like kvm_private_fd_get_pfn(), where the underlying ops struct can point
at any compatible backing store.

https://lore.kernel.org/all/YcuMUemyBXFYyxCC@google.com/

> Further, though mapped to a private gfn, it might be ok for QEMU to
> access the device fd in hva-based way (or call it MMU access way, e.g.
> read/write/mmap), it's desired that it could use the traditional to get
> pfn without convert the range to a shared one.

No, this is expressly forbidden.  The backing store for a private gfn must not
be accessible by userspace.  It's possible a backing store could support both, but
not concurrently, and any conversion must be done without KVM being involved.
In other words, resolving a private gfn must either succeed or fail (exit to
userspace), KVM cannot initiate any conversions.

> pfn = __gfn_to_pfn_memslot(slot, fault->gfn, ...)
> 	|->addr = __gfn_to_hva_many (slot, gfn,...)
> 	|  pfn = hva_to_pfn (addr,...)
> 
> 
> So, is it possible to recognize such kind of backends in KVM, and to get
> the pfn in traditional way without converting them to shared?
> e.g.
> - specify KVM_MEM_PRIVATE_NONPROTECT to memory regions with such kind
> of backends, or
> - detect the fd type and check if get_pfn is provided. if no, go the
>   traditional way.

No, because the whole point of this is to make guest private memory inaccessible
to host userspace.  Or did I misinterpret your questions?