movdqu partial write at page boundary

Bug #1785734 reported by Fabian
This bug affects 2 people
Affects Status Importance Assigned to Milestone

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
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

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

Revision history for this message
Fabian (fahe) wrote :
description: updated
Revision history for this message
Richard Henderson (rth) wrote :

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.

pranith (bobby-prani)
Changed in qemu:
status: New → Confirmed
Revision history for this message
Thomas Huth (th-huth) wrote :

The QEMU project is currently moving its bug tracking to another system.
For this we need to know which bugs are still valid and which could be
closed already. Thus we are setting the bug state to "Incomplete" now.

If the bug has already been fixed in the latest upstream version of QEMU,
then please close this ticket as "Fix released".

If it is not fixed yet and you think that this bug report here is still
valid, then you have two options:

1) If you already have an account on, please open a new ticket
for this problem in our new tracker here:

and then close this ticket here on Launchpad (or let it expire auto-
matically after 60 days). Please mention the URL of this bug ticket on
Launchpad in the new ticket on GitLab.

2) If you don't have an account on and don't intend to get
one, but still would like to keep this ticket opened, then please switch
the state back to "New" or "Confirmed" within the next 60 days (other-
wise it will get closed as "Expired"). We will then eventually migrate
the ticket automatically to the new system (but you won't be the reporter
of the bug in the new system and thus you won't get notified on changes

Thank you and sorry for the inconvenience.

Changed in qemu:
status: Confirmed → Incomplete
Revision history for this message
Launchpad Janitor (janitor) wrote :

[Expired for QEMU because there has been no activity for 60 days.]

Changed in qemu:
status: Incomplete → Expired
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