time: fix typo that makes sync_cmos_clock erratic
Subsystems (6)
Re: [PATCH] x86_64: fix problems due to use of "outb" to port 80 on some AMD64x2 laptops, etc.
In reply to David P. ReedQuoted 5 lines · David P. Reed
Quoted 4 lines · Avi Kivity
kvm will forward a virtual machine's writes to port
0x80to the real port. The reason is that the write is much faster than exiting and emulating it; the difference is measurable when compiling kernels.
Quoted 8 lines
Quoted 5 lines
Now if the cause is simply writing to port
0x80, then we must stop doing that. But if the reason is the back-to-back writes, when we can keep it, since the other writes will be trapped by kvm and emulated. Do you which is the case?As for kvm, I don't have enough info to know anything about that. Is there a test you'd like me to try?
I have a test, but I see that it is broken for mainline. I'll update it eventually, but...
Quoted 8 lines
I think you are also asking if the crash on these laptops is caused only by back-to-back writes. Actually, it doesn't seem to matter if they are back to back. I can cause the crash if the writes to 80 are very much spread out in time - it seems only to matter how many of them get executed - almost as if there is a buffer overflow. (And of course if you do back to back writes to other ports that are apparently fully unused, such as
0xEDon my machine, no crash occurs).
Quoted 4 lines
I believe (though no one seems to have confirming documentation from the chipset or motherboard vendor) that port 80 is actually functional
for some unknown function on these machines. (They do respond to
"in" instructions faster than a bus cycle abort does - more evidence).
That seems to be sufficient evidence for me to remove port 0x80 pass-through from kvm and emulate it instead. Given that port 80 writes take 1 microsecond, and that an in-kernel exit handler takes a similar amount of time, there won't be any significant performance loss.
Signature · 9 lines
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