Il 20/03/2014 00:27, Grigory Makarevich ha scritto:
Hi All,
I have been exploring different ways to implement on-demand paging for
VMs running in KVM.
The core of the idea is to introduce an additional exit
KVM_EXIT_MEMORY_NOT_PRESENT to inform VMM's user space to process
access to "not yet present" guest's page.
Each memory slot may be instructed to keep track of ondemand bit per
page. If the page is marked as "ondemand", page fault will generate
exit to the host's
user-space with the information about the faulting page. Once the page
is filled, VMM instructs the KVM to clear "ondemand" bit for the page.
I have working prototype and would like to consider upstreaming
corresponding KVM changes.
To start up the discussion before sending the actual patch-set, I'd like
to send the patch for the kvm's api.txt. Please, let me know what you
think.
Hi, Andrea Arcangeli is considering a similar infrastructure at the
generic mm level. Last time I discussed it with him, his idea was
roughly to have:
* a "userfaultfd" syscall that would take a memory range and return a
file descriptor; the file descriptor becomes readable when the first
access happens on a page in the region, and the read gives the address
of the access. Any thread that accesses a still-unmapped region remains
blocked until the address of the faulting page is written back to the
userfaultfd, or gets a SIGBUS if the userfaultfd is closed.
* a remap_anon_pages syscall that would be used in the userfaultfd I/O
handler to make the page accessible. The handler would build the page
in a "shadow" area with the actual contents of guest memory, and then
remap the shadow area onto the actual guest memory.
Andrea, please correct me.
QEMU would use this infrastructure for post-copy migration and possibly
also for live snapshotting of the guests. The advantage in making this
generic rather than KVM-based is that QEMU could use it also in
system-emulation mode (and of course anything else needing a read
barrier could use it too).
Paolo
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