[Qemu-devel] [Bug 1785734] Re: movdqu partial write at page boundary

[Richard Henderson]

This is a part of a class of related problems for qemu linux-user, in
that any non-atomic store is not validated before initiating a partial
write.

For instance, qemu-x86_64, built for arm32, would show this same partial
store problem for any 64-bit write crossing a page boundary because we
are forced by the limits of the host to split the store into two 32-bit
pieces.

While we could probably fix this specific case fairly easily, because it
is implemented with an external helper in the first place, we would need
some new infrastructure to handle the more general problem.  Exactly
what form that should take would need some thought and discussion.

-- 
You received this bug notification because you are a member of qemu-
devel-ml, which is subscribed to QEMU.
https://bugs.launchpad.net/bugs/1785734

Title:
  movdqu partial write at page boundary

Status in QEMU:
  New

Bug description:
  In TCG mode, when a 16-byte write instruction (such as movdqu) is
  executed at a page boundary and causes a page fault, a partial write
  is executed in the first page. See the attached code for an example.

  Tested on the qemu-3.0.0-rc1 release.

  % gcc -m32 qemu-bug2.c && ./a.out && echo && qemu-i386 ./a.out
  [snip]
  page fault: addr=0x70001000 err=0x7
  *(0x70000ff8+ 0) = aa
  *(0x70000ff8+ 1) = aa
  *(0x70000ff8+ 2) = aa
  *(0x70000ff8+ 3) = aa
  *(0x70000ff8+ 4) = aa
  *(0x70000ff8+ 5) = aa
  *(0x70000ff8+ 6) = aa
  *(0x70000ff8+ 7) = aa
  *(0x70000ff8+ 8) = 55
  *(0x70000ff8+ 9) = 55
  *(0x70000ff8+10) = 55
  *(0x70000ff8+11) = 55
  *(0x70000ff8+12) = 55
  *(0x70000ff8+13) = 55
  *(0x70000ff8+14) = 55
  *(0x70000ff8+15) = 55

  [snip]
  page fault: addr=0x70001000 err=0x6
  *(0x70000ff8+ 0) = 77
  *(0x70000ff8+ 1) = 66
  *(0x70000ff8+ 2) = 55
  *(0x70000ff8+ 3) = 44
  *(0x70000ff8+ 4) = 33
  *(0x70000ff8+ 5) = 22
  *(0x70000ff8+ 6) = 11
  *(0x70000ff8+ 7) = 0
  *(0x70000ff8+ 8) = 55
  *(0x70000ff8+ 9) = 55
  *(0x70000ff8+10) = 55
  *(0x70000ff8+11) = 55
  *(0x70000ff8+12) = 55
  *(0x70000ff8+13) = 55
  *(0x70000ff8+14) = 55
  *(0x70000ff8+15) = 55

To manage notifications about this bug go to:
https://bugs.launchpad.net/qemu/+bug/1785734/+subscriptions