Re: [PATCH 1/2] KVM: Implement kvm_copy_guest() to perform copy of guest memory in place

[Suraj Jitindar Singh <sjitindarsingh@gmail.com>]

On Wed, 2019-03-06 at 09:21 -0800, Sean Christopherson wrote:
> On Wed, Mar 06, 2019 at 05:00:15PM +1100, Suraj Jitindar Singh wrote:
> > Implement the function kvm_copy_guest() to be used to perform a
> > memory
> > copy from one guest physical address to another of a variable
> > length.
> > 
> > This performs similar functionality as the kvm_read_guest() and
> > kvm_write_guest() functions, except both addresses point to guest
> > memory.
> > This performs a copy in place using raw_copy_in_user() to avoid
> > having to
> > buffer the data.
> > 
> > The guest memory can reside in different memslots and the copy
> > length
> > can span memslots.
> > 
> > Signed-off-by: Suraj Jitindar Singh <sjitindarsingh@gmail.com>
> > 
> > ---
> > 
> > I suspect additional checking may be required around the
> > raw_copy_in_user()
> > call.
> > 
> > ---
> >  include/linux/kvm_host.h |  1 +
> >  virt/kvm/kvm_main.c      | 69
> > ++++++++++++++++++++++++++++++++++++++++++++++++
> >  2 files changed, 70 insertions(+)
> > 
> 
> ...
> 
> > +static int next_segment_many(unsigned long len, int offset1, int
> > offset2)
> > +{
> > +	int size = min(PAGE_SIZE - offset1, PAGE_SIZE - offset2);
> > +
> > +	if (len > size)
> > +		return size;
> > +	else
> > +		return len;
> > +}
> > +
> > +int kvm_copy_guest(struct kvm *kvm, gpa_t to, gpa_t from, unsigned
> > long len)
> > +{
> > +	struct kvm_memory_slot *to_memslot = NULL;
> > +	struct kvm_memory_slot *from_memslot = NULL;
> > +	gfn_t to_gfn = to >> PAGE_SHIFT;
> > +	gfn_t from_gfn = from >> PAGE_SHIFT;
> > +	int seg;
> > +	int to_offset = offset_in_page(to);
> > +	int from_offset = offset_in_page(from);
> > +	int ret;
> > +
> > +	while ((seg = next_segment_many(len, to_offset,
> > from_offset)) != 0) {
> > +		if (!to_memslot || (to_gfn >= (to_memslot-
> > >base_gfn +
> > +					       to_memslot-
> > >npages)))
> > +			to_memslot = gfn_to_memslot(kvm, to_gfn);
> > +		if (!from_memslot || (from_gfn >= (from_memslot-
> > >base_gfn +
> > +						   from_memslot-
> > >npages)))
> > +			from_memslot = gfn_to_memslot(kvm,
> > from_gfn);
> > +
> > +		ret = __kvm_copy_guest_page(to_memslot, to_gfn,
> > to_offset,
> > +					    from_memslot,
> > from_gfn, from_offset,
> > +					    seg);
> > +		if (ret < 0)
> > +			return ret;
> > +
> > +		to_offset = (to_offset + seg) & (PAGE_SIZE - 1);
> > +		from_offset = (from_offset + seg) & (PAGE_SIZE -
> > 1);
> > +		len -= seg;
> > +		if (!to_offset)
> > +			to_gfn += 1;
> > +		if (!from_offset)
> > +			from_gfn += 1;
> > +	}
> > +	return 0;
> > +}
> > +EXPORT_SYMBOL_GPL(kvm_copy_guest);
> 
> Is there a need to support spanning multiple pages at this
> time?  Your use
> case always accesses exactly a page and requires both dst and src to
> be
> page aligned.  I.e. provide just kvm_copy_guest_page() for
> simplicity.

There is no need for multiple pages at this stage. However I wanted to
match kvm_[read/write]_guest which allow multiple pages and there
didn't seem any reason to not just implement it this way from the
start.

> 
> > +
> >  static int __kvm_gfn_to_hva_cache_init(struct kvm_memslots *slots,
> >  				       struct gfn_to_hva_cache
> > *ghc,
> >  				       gpa_t gpa, unsigned long
> > len)
> > -- 
> > 2.13.6
> >