R. Armiento wrote:
Is this hack really 'safe'? I don't claim to know much about modern
x86 instructions, but some googling tells me that mwait is supposed
to wake on a monitored memory write (but is allowed to wake up
earlier, hence it is acceptable but CPU consuming to emulate it
with a
R. Armiento wrote:
Couldn't there be situations where someone depends on mwait waking up
without there being an event that wakes hlt? Or are we sure qemu's hlt
will happen to wake up anyway?
Joachim Henke wrote:
Currently the Linux kernel simply uses monitor/mwait as a faster 'hlt'
Problem is, at the moment I've no idea, how we could achieve this memory
monitoring in a safe and simple way in user space.
I'm trying to read up on monitor and mwait. Apparently mwait puts the
processor in low-power wait mode, waiting for a memory write in some
select area defined by
Joachim Henke wrote:
Currently the Linux kernel simply uses monitor/mwait as a faster
'hlt' replacement, so it should be safe there. I don't know about
other guest OSs. Anyway, I proposed this hack only as a quick
solution for local usage.
As long as there's only one CPU and 'mwait'
R. Armiento wrote:
I'm not sure if I have understood all sources from where such a
memory write can come from while the processor is asleep. One
source, I suppose, is from other processors in an SMP setup? Another
source may be DMA? Does this mean that it is safe to emulate wmait
as hlt if
Paul Brook wrote:
qemu hardware does support DMA, but I don't think this matters.
By my reading DMA writes don't need to wake mwait.
The exact wording is store operation, which I'd expect to mean
execution of a store instruction (by a different CPU).
Hmm. x86 CPUs snoop writes by DMA as well
R. Armiento wrote:
However, as you probably know, despite not declaring MONITOR in
qemu, kqemu sees MONITOR on the host processor and Linux CPU usage
will still be 100%, even with your updated patch. So even with your
patch applied one should use the 'idle=halt' kernel parameter when
Hi,
Again, thank you for helping out with updated patches, it is much
appreciated.
Joachim Henke wrote:
R. Armiento wrote:
So even with your patch applied one should use the 'idle=halt'
kernel parameter when booting Linux with -kernel-kqemu on newer
processors. [...]
To lower the cpu
