Re: [kvm-devel] [RFC 0/8]KVM: swap out guest pages

[Avi Kivity <avi@qumranet.com>]

Rusty Russell wrote:
> On Mon, 2007-07-23 at 13:27 +0300, Avi Kivity wrote:
>   
>> Having an address_space (like your patch does) is remarkably simple, and 
>> requires few hooks from the current vm.  However using existing vmas 
>> mapped by the user has many advantages:
>>
>> - compatible with s390 requirements
>> - allows the user to use hugetlbfs pages, which have a performance 
>> advantage using ept/npt (but which are unswappable)
>> - allows the user to map a file (which can be regarded as way to specify 
>> the swap device)
>> - better ingration with the rest of the vm
>>     
>
> You don't need to expose the vmas.  You just have userspace point out
> the start+len of each region of memory it wants the guest to be able to
> access, and the address it wants it to appear in the guest.
>
> This is a slight superset of what lguest does in two ways:
>
> 1) my guest address == user address, but I'm looking at adding an offset
> so I don't have to link the launcher binary specially.
> 2) I have only one contiguous region of guest-physical memory, since I
> can place device memory immediately above "normal" mem.
>
>   

My intent was to allow userspace to establish assign a virtual address
range into a memory slot.

So long as you don't do swapping, all is simple, since you can do a
get_user_pages() on initialization or when installing a shadow pte.  But
if you want to swap, you need:

- a way to transfer the dirty bit from the shadow ptes to the struct page
- a way to let the vm rmap know that there are shadow ptes that point to
the page in addition to Linux ptes.  These shadow ptes may be in a
different format than Linux ptes.
- a different tlb invalidation method with ASIDs

It's not going to be simple.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.