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

[Yan Zhao]

On Wed, Jan 05, 2022 at 02:28:10PM +0800, Chao Peng wrote:
> On Tue, Jan 04, 2022 at 06:06:12PM +0800, Yan Zhao wrote:
> > On Tue, Jan 04, 2022 at 05:10:08PM +0800, Chao Peng wrote:
<...> 
> > Thanks. So QEMU will re-generate memslots and set KVM_MEM_PRIVATE
> > accordingly? Will it involve slot deletion and create?
> 
> KVM will not re-generate memslots when do the conversion, instead, it
> does unmap/map a range on the same memslot. For memslot with tag
> KVM_MEM_PRIVATE, it always have two mappings (private/shared) but at a
> time only one is effective. What conversion does is to turn off the
> existing mapping and turn on the other mapping for specified range in
> that slot.
>
got it. thanks!

<...>
> > > > > +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);
> > > > For private memory slots, it's possible to have pfns backed by
> > > > backends other than memfd, e.g. devicefd.
> > > 
> > > Surely yes, although this patch only supports memfd, but it's designed
> > > to be extensible to support other memory backing stores than memfd. There
> > > is one assumption in this design however: one private memslot can be
> > > backed by only one type of such memory backing store, e.g. if the
> > > devicefd you mentioned can independently provide memory for a memslot
> > > then that's no issue.
> > > 
> > > >So is it possible to let those
> > > > private memslots keep private and use traditional hva-based way?
> > > 
> > > Typically this fd-based private memory uses the 'offset' as the
> > > userspace address to get a pfn from the backing store fd. But I believe
> > > the current code does not prevent you from using the hva as the
> > By hva-based way, I mean mmap is required for this fd.
> > 
> > > userspace address, as long as your memory backing store understand that
> > > address and can provide the pfn basing on it. But since you already have
> > > the hva, you probably already mmap-ed the fd to userspace, that seems
> > > not this private memory patch can protect you. Probably I didn't quite
> > Yes, for this fd, though mapped in private memslot, there's no need to
> > prevent QEMU/host from accessing it as it will not cause the severe machine
> > check.
> > 
> > > understand 'keep private' you mentioned here.
> > 'keep private' means allow this kind of private memslot which does not
> > require protection from this private memory patch :)
> 
> Then I think such memory can be the shared part of memory of the
> KVM_MEM_PRIVATE memslot. As said above, this is initially supported :)
>
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.

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.
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.

Thanks
Yan

> > > > Reasons below:
> > > > 1. only memfd is supported in this patch set.
> > > > 2. qemu/host read/write to those private memslots backing up by devicefd may
> > > > not cause machine check.