mirror of https://github.com/xemu-project/xemu.git
fc78d5ee76
# Background I was investigating spurious non-deterministic EINTR returns from various 9p file system operations in a Linux guest served from the qemu 9p server. ## EINTR, ERESTARTSYS and the linux kernel When a signal arrives that the Linux kernel needs to deliver to user-space while a given thread is blocked (in the 9p case waiting for a reply to its request in 9p_client_rpc -> wait_event_interruptible), it asks whatever driver is currently running to abort its current operation (in the 9p case causing the submission of a TFLUSH message) and return to user space. In these situations, the error message reported is generally ERESTARTSYS. If the userspace processes specified SA_RESTART, this means that the system call will get restarted upon completion of the signal handler delivery (assuming the signal handler doesn't modify the process state in complicated ways not relevant here). If SA_RESTART is not specified, ERESTARTSYS gets translated to EINTR and user space is expected to handle the restart itself. ## The 9p TFLUSH command The 9p TFLUSH commands requests that the server abort an ongoing operation. The man page [1] specifies: ``` If it recognizes oldtag as the tag of a pending transaction, it should abort any pending response and discard that tag. [...] When the client sends a Tflush, it must wait to receive the corresponding Rflush before reusing oldtag for subsequent messages. If a response to the flushed request is received before the Rflush, the client must honor the response as if it had not been flushed, since the completed request may signify a state change in the server ``` In particular, this means that the server must not send a reply with the orignal tag in response to the cancellation request, because the client is obligated to interpret such a reply as a coincidental reply to the original request. # The bug When qemu receives a TFlush request, it sets the `cancelled` flag on the relevant pdu. This flag is periodically checked, e.g. in `v9fs_co_name_to_path`, and if set, the operation is aborted and the error is set to EINTR. However, the server then violates the spec, by returning to the client an Rerror response, rather than discarding the message entirely. As a result, the client is required to assume that said Rerror response is a result of the original request, not a result of the cancellation and thus passes the EINTR error back to user space. This is not the worst thing it could do, however as discussed above, the correct error code would have been ERESTARTSYS, such that user space programs with SA_RESTART set get correctly restarted upon completion of the signal handler. Instead, such programs get spurious EINTR results that they were not expecting to handle. It should be noted that there are plenty of user space programs that do not set SA_RESTART and do not correctly handle EINTR either. However, that is then a userspace bug. It should also be noted that this bug has been mitigated by a recent commit to the Linux kernel [2], which essentially prevents the kernel from sending Tflush requests unless the process is about to die (in which case the process likely doesn't care about the response). Nevertheless, for older kernels and to comply with the spec, I believe this change is beneficial. # Implementation The fix is fairly simple, just skipping notification of a reply if the pdu was previously cancelled. We do however, also notify the transport layer that we're doing this, so it can clean up any resources it may be holding. I also added a new trace event to distinguish operations that caused an error reply from those that were cancelled. One complication is that we only omit sending the message on EINTR errors in order to avoid confusing the rest of the code (which may assume that a client knows about a fid if it sucessfully passed it off to pud_complete without checking for cancellation status). This does mean that if the server acts upon the cancellation flag, it always needs to set err to EINTR. I believe this is true of the current code. [1] https://9fans.github.io/plan9port/man/man9/flush.html [2] https://github.com/torvalds/linux/commit/9523feac272ccad2ad8186ba4fcc891 Signed-off-by: Keno Fischer <keno@juliacomputing.com> Reviewed-by: Greg Kurz <groug@kaod.org> [groug, send a zero-sized reply instead of detaching the buffer] Signed-off-by: Greg Kurz <groug@kaod.org> Acked-by: Michael S. Tsirkin <mst@redhat.com> Reviewed-by: Stefano Stabellini <sstabellini@kernel.org> |
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README
QEMU README
===========
QEMU is a generic and open source machine & userspace emulator and
virtualizer.
QEMU is capable of emulating a complete machine in software without any
need for hardware virtualization support. By using dynamic translation,
it achieves very good performance. QEMU can also integrate with the Xen
and KVM hypervisors to provide emulated hardware while allowing the
hypervisor to manage the CPU. With hypervisor support, QEMU can achieve
near native performance for CPUs. When QEMU emulates CPUs directly it is
capable of running operating systems made for one machine (e.g. an ARMv7
board) on a different machine (e.g. an x86_64 PC board).
QEMU is also capable of providing userspace API virtualization for Linux
and BSD kernel interfaces. This allows binaries compiled against one
architecture ABI (e.g. the Linux PPC64 ABI) to be run on a host using a
different architecture ABI (e.g. the Linux x86_64 ABI). This does not
involve any hardware emulation, simply CPU and syscall emulation.
QEMU aims to fit into a variety of use cases. It can be invoked directly
by users wishing to have full control over its behaviour and settings.
It also aims to facilitate integration into higher level management
layers, by providing a stable command line interface and monitor API.
It is commonly invoked indirectly via the libvirt library when using
open source applications such as oVirt, OpenStack and virt-manager.
QEMU as a whole is released under the GNU General Public License,
version 2. For full licensing details, consult the LICENSE file.
Building
========
QEMU is multi-platform software intended to be buildable on all modern
Linux platforms, OS-X, Win32 (via the Mingw64 toolchain) and a variety
of other UNIX targets. The simple steps to build QEMU are:
mkdir build
cd build
../configure
make
Additional information can also be found online via the QEMU website:
https://qemu.org/Hosts/Linux
https://qemu.org/Hosts/Mac
https://qemu.org/Hosts/W32
Submitting patches
==================
The QEMU source code is maintained under the GIT version control system.
git clone git://git.qemu.org/qemu.git
When submitting patches, the preferred approach is to use 'git
format-patch' and/or 'git send-email' to format & send the mail to the
qemu-devel@nongnu.org mailing list. All patches submitted must contain
a 'Signed-off-by' line from the author. Patches should follow the
guidelines set out in the HACKING and CODING_STYLE files.
Additional information on submitting patches can be found online via
the QEMU website
https://qemu.org/Contribute/SubmitAPatch
https://qemu.org/Contribute/TrivialPatches
Bug reporting
=============
The QEMU project uses Launchpad as its primary upstream bug tracker. Bugs
found when running code built from QEMU git or upstream released sources
should be reported via:
https://bugs.launchpad.net/qemu/
If using QEMU via an operating system vendor pre-built binary package, it
is preferable to report bugs to the vendor's own bug tracker first. If
the bug is also known to affect latest upstream code, it can also be
reported via launchpad.
For additional information on bug reporting consult:
https://qemu.org/Contribute/ReportABug
Contact
=======
The QEMU community can be contacted in a number of ways, with the two
main methods being email and IRC
- qemu-devel@nongnu.org
https://lists.nongnu.org/mailman/listinfo/qemu-devel
- #qemu on irc.oftc.net
Information on additional methods of contacting the community can be
found online via the QEMU website:
https://qemu.org/Contribute/StartHere
-- End