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[Xen-devel] Re: [RFC, PATCH 5/24] i386 Vmi code patching

Pavel Machek wrote:
The question of licensing of such ROM code is a completely separate
issue.  We are not trying to hide some proprietary code by putting it
inside of a ROM to keep it hidden.  In fact, you can disassemble the
ROM code and see it quite readily - and you know all of the entry

Could you disassemble one entry point for us and describe how it works?

000005e0 <stub_VMI_SwapPxE>:
    5e0:       e9 db 06 00 00          jmp    cc0 <VMI_SwapPxE>
5e5: 0f 0b ud2a
(The SwapPxE call does not fit in the 32-byte region).

00000cc0 <VMI_SwapPxE>:
    cc0:       83 ec 1c                sub    $0x1c,%esp
    cc3:       89 c1                   mov    %eax,%ecx
    cc5:       89 74 24 10             mov    %esi,0x10(%esp,1)
    cc9:       89 d6                   mov    %edx,%esi
    ccb:       89 7c 24 14             mov    %edi,0x14(%esp,1)
    ccf:       89 6c 24 18             mov    %ebp,0x18(%esp,1)
    cd3:       89 54 24 0c             mov    %edx,0xc(%esp,1)

(Save registers and call values on stack)

    cd7:       87 30                   xchg   %esi,(%eax)

(Atomically swap the PTE in the page)

    cd9:       0f b6 44 24 0c          movzbl 0xc(%esp,1),%eax

(Extract low byte of the PTE write just done)

    cde:       31 d2                   xor    %edx,%edx
    ce0:       09 f0                   or     %esi,%eax
    ce2:       a8 01                   test   $0x1,%al

(Were either the new or old PTE's present?)

    ce4:       74 11                   je     cf7 <VMI_SwapPxE+0x37>

(No?  jump forward to branch combination)

    ce6:       0f b6 44 24 0c          movzbl 0xc(%esp,1),%eax

(Extract low byte of the PTE write again)

    ceb:       31 f0                   xor    %esi,%eax

(Compute changed bits in PTE)

    ced:       a8 c7                   test   $0xc7,%al

(Have U/S, R/W, P, PS, D bits changed?)

    cef:       b8 01 00 00 00          mov    $0x1,%eax
    cf4:       0f 45 d0                cmovne %eax,%edx
    cf7:       84 d2                   test   %dl,%dl

(Combine above tests into one branch)

    cf9:       74 32                   je     d2d <VMI_SwapPxE+0x6d>

(If nothing interesting changed, return)

    cfb:       b8 54 00 00 fc          mov    $0xfc000054,%eax

(Load start of linear address translation lookaside)

    d00:       31 d2                   xor    %edx,%edx
    d02:       89 cf                   mov    %ecx,%edi
    d04:       39 08                   cmp    %ecx,(%eax)

(Compare stored lookaside translation address with address of PTE write)

    d06:       77 37                   ja     d3f <VMI_SwapPxE+0x7f>

(Greater? - next)

    d08:       39 48 04                cmp    %ecx,0x4(%eax)

(Compare stored lookaside translate address end with address of PTE write)

    d0b:       76 32                   jbe    d3f <VMI_SwapPxE+0x7f>

(Below/Equal? - next)

    d0d:       03 78 08                add    0x8(%eax),%edi

(Add translation offset to PTE address (perform VA->PA conversion))

    d10:       c7 04 24 00 00 00 00    movl   $0x0,(%esp,1)
    d17:       8b 54 24 0c             mov    0xc(%esp,1),%edx
    d1b:       bd 89 00 00 00          mov    $0x89,%ebp
    d20:       89 6c 24 04             mov    %ebp,0x4(%esp,1)
    d24:       31 c9                   xor    %ecx,%ecx
    d26:       89 f8                   mov    %edi,%eax
    d28:       e8 73 fd ff ff          call   aa0 <VMIEnqueue>

(Put arguments into order, and queue the PTE update)

    d2d:       89 f0                   mov    %esi,%eax
    d2f:       8b 74 24 10             mov    0x10(%esp,1),%esi
    d33:       8b 7c 24 14             mov    0x14(%esp,1),%edi
    d37:       8b 6c 24 18             mov    0x18(%esp,1),%ebp
    d3b:       83 c4 1c                add    $0x1c,%esp
    d3e:       c3                      ret

(Restore registers and return)

    d3f:       42                      inc    %edx
    d40:       83 c0 10                add    $0x10,%eax

(Move to next translation index)

    d43:       83 fa 03                cmp    $0x3,%edx
    d46:       7e bc                   jle    d04 <VMI_SwapPxE+0x44>


    d48:       89 cf                   mov    %ecx,%edi
    d4a:       eb c4                   jmp    d10 <VMI_SwapPxE+0x50>

(Default case - VA == PA, direct mapping assumed)

And this version may be easier for some of you to read:

* To convert virtual address to physical address, we must
* keep a cache of currently mapped page tables.  This allows
* us to perform the VA->PA conversion with guest help, which
* is much faster than having to do a page table walk.  For the
* Linux guest, when page tables are not in high memory, the
* first mapping slot is always an identity map of physical
* memory mapped at PAGE_OFFSET.

static INLINE PA
VMITranslate(const LA la)
{ HyperAddressMap *m;
  int i;
  m = &vmiHypervisorShared->pageTranslation[0];
  for (i = 0; i < VMI_LINEAR_MAP_SLOTS; i++, m++) {
     if (LIKELY(la >= m->startVA && la < m->endVA)) {
        return (m->physOffset + la);
  /* Assume identity mapped */
  return la;

* Note we read and write guest memory directly.  This
* is safe for us because we use shadow page tables.  The
* Xen ROM would perform similar conditional tests here to
* figure out which hypercall it wants to make or decide
* to write the PTE directly and trigger shadow

VMICALL(SwapPxE, VM_PTE *const ptep, VM_PTE const pteval)
  VM_PTE pteold;
pteold = Atomic_ReadWrite((Atomic_uint32 *)ptep, pteval);
  if (VMIIsPTEInteresting(pteval, pteold)) {
     PA pa = VMITranslate((LA) ptep);
     VMIEnqueue(pa, pteval, 0, 0, HC_SetPxEAsync);

Herein lies one of our problems. Atomic_ReadWrite comes from a private header file. We need to copy all of these defines and inline functions into a distributable header file rather than a private one - separating public and private headers can be sort of like trying to straighten the vines of a giant thornbush. Every time you straighten one, another wacks you in the head. We have many, many build clients that depend on those same headers, and moving large amounts of things around is a tricky and painstaking process. Not that it can't be done, just that it is not a simple copy and paste job.


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