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. > 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 ?) Tristan.
