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[Xen-devel] Re: [PATCH]Further release shadow lock overhead

At 21:23 +0800 on 22 Jan (1201037017), Tian, Kevin wrote:
> Anyway, my points are:
>       Shadow lock is just the lock for shadow page tables,
> which can't be used to prevent guest page table race. Even
> when guest table walk is included in shadow lock, there's no
> determined result for insane guest behavior.

Agreed.  I'm willing to believe that the walks can be done without the
lock held (just didn't want to assert it was safe without finding time
to look more closely).

But the current semantics of the shadow lock also includes maintaining
the invariant that a shadow entry is always a valid shadow of the guest
entry it represents (or not present). 

It's relied on for level-2 and higher entries in the pagefault handler
-- if there is a shadow pagetable present mapping a particular VA range,
it is assumed that it is the shadow of the guest pagetable.  For
example, using a 2-level system for simplicity:

- Guest l2e in slot 0 is 0x00001067 (GFN 0x1 is the gl1 table for VA 0x0)
  Shadow l2e in slot 0 is 0x0000a067 (MFN 0xA is the sl1 table for VA 0x0) 

- Guest vcpu 1 writes 0x00002067 into l2e slot 0.  This write is
  emulated, and the shadow lock is not held when the guest pagetable is
  written to in sh_x86_emulate_write().  Just before the lock is taken...

- Guest vcpu 2 takes a pagefault to VA 0x0 (and is using the same
  pagetable as vcpu 1 -- unlikely but possible, and less crazy scenarios
  have the same effect on 4-level pagetables).

- The shadow fault handler sees 0x00002067, and maps GFN 0x2 as the
  guest l1 table.  It sees that there is already a shadow l1 table
  present; so it propagates the contents of an l1 from GFN 0x2 into MFN
  0xA, which is the wrong shadow.

- Guest vcpu 1's emulation now acquires the shadow lock and updates the
  shadow l2e to 0x0000B067, but it's too late - the shadows are already
  corrupted, and no amount of TLB flushing will make it better.

Sorry if that's a bit long-winded; needed to make sure it was still true
after the recent rework of the shadow PT walker.



Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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