On 02/08/2011 05:46 AM, Aurelien Jarno wrote:
On Tue, Feb 08, 2011 at 12:07:02PM +0100, Tristan Gingold wrote:
On Feb 8, 2011, at 6:58 PM, Anthony Liguori wrote:
On 02/08/2011 04:06 AM, Aurelien Jarno wrote:
Yes, it's slow. But is it a problem? You assume that people use QEMU
only for emulating SMP platforms. This is a wrong assumption. Beside the
x86 target, only sparc really supports SMP emulation.
It's *not* just about performance.
TCG requires a signal to break out of a tight chained TB loop. If you have a
guest in a tight loop waiting for something external (like polling on a
in-memory flag), the device emulation will not get to run until a signal is
fired.
Unless you set SIGIO on every file descriptor that selects polls on (and you
can't because there are a number that just don't support SIGIO), then you have
a race condition.
A race condition ? Looks like you are describing a dead-lock.
But the dead lock doesn't happen because of the timer which periodically exits
from TCG. Hence the performance issue.
With dynticks, you don't always have a periodic timer (unless the guest
has a periodic timer enabled). There's a good bit of early startup code
that runs without a periodic timer enabled.
Now that said, we never truly sleep forever. We'll set something like a
5 second timeout. But 5 seconds might as well be forever and this is
certainly a giant hack.
Regards,
Anthony Liguori
This can be fixed by running TCG in a separate thread than select() and sending
a signal to the TCG VCPU when select() returns (effectively SIGIO in userspace).
This is exactly what the I/O thread does.
(Nobody was able to make it working on Windows - or nobody was interested in ?)
Given the I/O thread is disabled by default, my guess is that nobody
really see an interest in looking at that.
