KVM

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Revision as of 14:51, 11 July 2012; view current revision
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ToDo

The following items need some love. Please post to the list if you are interested in helping out:

  • Emulate MSR_IA32_DEBUGCTL for guests which use it
  • Bring up Windows 95 and Windows 98 guests
  • Implement ACPI memory hotplug
  • Improve ballooning to try to use 2MB pages when possible ( in progress - kern.devel@gmail.com )


MMU related:

  • Improve mmu page eviction algorithm (currently FIFO, change to approximate LRU).
  • Add a read-only memory type.
    • possible using mprotect()?
  • Implement AM20 for dos and the like.
  • O(1) write protection by protecting the PML4Es, then on demand PDPTEs, PDEs, and PTEs
  • Simpler variant: don't drop large ptes when write protecting; just write protect them. When taking a write fault, either drop the large pte, or convert it to small ptes and write protect those (like O(1) write protection).
  • O(1) mmu invalidation using a generation number

x86 emulator updates:

  • On-demand register access, really, copying all registers all the time is gross.
    • Can be done by adding 'available' and 'dirty' bitmasks
  • Implement mmx and sse memory move instructions; useful for guests that use multimedia extensions for accessing vga (partially done)
  • Implement an operation queue for the emulator. The emulator often calls userspace to perform a read or a write, but due to inversion of control it actually restarts instead of continuing. The queue would allow it to replay all previous operations until it reaches the point it last stopped.
    • if this is done, we can retire ->read_std() in favour of ->read_emulated().
  • convert more instructions to direct dispatch (function pointer in decode table)
  • move init_emulate_ctxt() into x86_decode_insn() and other emulator entry points

Interactivity improvements:

  • If for several frames in a row a large proportion of the framebuffer pages are changing, then for the next few frames don't bother to get the dirty page log from kvm, but instead assume that all pages are dirty. This will reduce page fault overhead on highly interactive workloads.
  • When detecting keyboard/video/mouse activity, scale up the frame rate; when activity dies down, scale it back down (applicable to qemu as well).

Pass-through/VT-d related:

  • Enhance KVM QEMU to return error messages if user attempts to pass-through unsupported devices:
    • Devices with shared host IOAPIC interrupt
    • Conventional PCI devices
    • Devices without FLR capability
  • QEMU PCI pass-through patch needs to be enhanced to same functionality as corresponding file in Xen
    • Remove direct HW access by QEMU for probing PCI BAR size
    • PCI handling of various PCI configuration registers
    • Other enhancements that was done in Xen
  • Host shared interrupt support
  • VT-d2 support (WIP in Linux Kernel)
    • Queued invalidation
    • Interrupt remapping
    • ATS
  • USB 2.0 (EHCI) support

Bug fixes:

  • Less sexy but ever important, fixing bugs is one of the most important contributions

Random improvements

  • Utilize the SVM interrupt queue to avoid extra exits when guest interrupts are disabled

For the adventurous:

  • Emulate the VMX instruction sets on qemu. This would be very beneficial to debugging kvm ( working on this - kern.devel@gmail.com ).
  • Add vmgl support to qemu. Port to virtio. Write a Windows driver.
  • Keep this TODO up to date

Nested VMX

  • Implement performance features such as EPT and VPID

KVM Safe Mode

An ioctl() from userspace that tells KVM to disable one or more of the following features:

  • shadow paging (force direct mapping)
  • instruction emulation (require virtio or mmio hypercall)
  • task switches
  • mode switches (long mode / legacy mode / real mode)
  • IDT/GDT/LDT changes
  • IDT/GDT/LDT write protect
  • write protect important MSRs (*STAR etc)

The idea is both to protect the guest from attacks, and to protect the host from the guest.


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