1. Shadow pagetables update
Tim Deegan, XenSource UK
© Copyright , XenSource, Inc. All rights reserved. 
Xen, XenSource, XenEnterprise, XenServer and XenExpress are either registered trademarks or trademarks of XenSource Inc. in the United States and/or other countries.
12/28/2018

2. Shadow (II) architecture
Shadow pages on demand
#PF, MOV CR3
Revoke guest write access to shadowed page
Unshadow by refcount, memory pressure, or bogus contents
Trap and emulate writes
Atomically update original and shadows
Per-domain “shadow lock”
12/28/2018

3. Don’t maintain shadows of PAE L3 tables
What’s changed?
Don’t maintain shadows of PAE L3 tables
Problems with shared-use pages
Complex “sub-shadow” mechanisms
Snapshot on MOV CR3 instead
Fast unshadow
Keep a single upward link in each shadow
Fast write-access revocation
Heuristics: guest’s linear maps, 1–1 maps
12/28/2018

4. Fast-path for MMIO, not-present
What’s changed?
Fast-path for MMIO, not-present
Use reserved bits in shadow PTEs
Handle these faults without taking the lock
Prefetching PTEs
Interface cleanup
Allow other “paging assistance” modes
Shadow lock now entirely private
p2m code split from shadow code
12/28/2018

5. (Office Productivity)
What’s changed?
SysMark
(Office Productivity)
A: Not optimized
B: Optimized
Most of the benefit in this benchmark is from the fast-path and prefetch
12/28/2018

6. Cache guest virt-to-phys translations
Coming soon…
Cache guest virt-to-phys translations
Faster lookups in MMIO + emulation
Invalidate on guest TLB flush or invlpg
Emulate-ahead in PAE mode
Guest that writes 32 bits to a PAE pagetable will soon write the “other half”
x86_emulate now complete enough to run through a few instructions in between
12/28/2018

7. Re-introduce out-of-sync shadows
Not so soon…
Re-introduce out-of-sync shadows
Only for pages with heavy churn
Spot many (~8) writes to the same page
Keep trap-and-emulate for demand-paging
Finer-grained shadow locking
Per-page locks and top-down ordering
Not until we know the shadow lock is the bottleneck
12/28/2018

8. Questions?
12/28/2018

9. Init/teardown of domains, vcpus. Domctl hypercall
Paging interface
Init/teardown of domains, vcpus.
Domctl hypercall
Mem alloc’n, mode changes, log-dirty ops
Calls for MMU programming events
#PF, MOV CR3, MOV CR0/4, INVLPG
Writes/cmpxchgs of pagetables
Writes to p2m
Reading/walking guest pagetables
12/28/2018