On Sat, 2009-03-14 at 16:45 -0700, Jeremy Fitzhardinge wrote:
Alok Kataria wrote:
Hi Peter,Nope. My first guess is that this is a misbisection, but the fact that reverting helps tends to undermine that diagnosis.
I was seeing a early fault when running tip/master with VMI enabled on
VMware platform. This early fault was in the vmi_patch code where we are applying
paravirt_alternatives. After some trials i noticed that this is
reproducible only with CONFIG_TRACING. With that disabled my VM boots
again.
I started a git bisect after that, and git pointed to this as the bad
commit
commit 6cc3c6e12bb039047974ad2e7e2d46d15a1b762f
trace_clock: fix preemption bug
I then reverted that commit from tip/master and the system did boot. But I fail to understand how this simple patch would be causing things
to fail in VMI. Any ideas ?
What crash are you seeing? What kind of fault? At what instruction?
It being a early fault, nothing is printed on the console the system
just stays stuck in this early_fault code in arch/x86/kernel/head_32.S
I don't understand why is this not printing anything at this early fault
though, the system just enters the hlt_loop and stays there.
Well, indirect "call *0xc13d875c". But yes, its hard to see what gcc thinks its getting out of doing the indirect call via %edx. We definitely don't want gcc doing such things, even if it does make sense (such as calling an op multiple times, and CSEing the pointer); given that we generate all this in asm anyway, we could force it. Does this help?Doing what? It's a bit hard to tell what you're actually seeing.I did some more debugging and I think i know what the problem is The objdump for trace_clock_local looks like this
c1070c24 <trace_clock_local>:
c1070c24: 55 push %ebp
c1070c25: 89 e5 mov %esp,%ebp
c1070c27: 53 push %ebx
c1070c28: 83 ec 08 sub $0x8,%esp
c1070c2b: ff 15 5c 87 3d c1 call *0xc13d875c
c1070c31: ba 64 87 3d c1 mov $0xc13d8764,%edx
c1070c36: 89 45 f4 mov %eax,0xfffffff4(%ebp)
c1070c39: ff 12 call *(%edx) <<=====*
c1070c3b: e8 cd 68 f9 ff call c100750d <sched_clock>
c1070c40: 89 c1 mov %eax,%ecx
c1070c42: 8b 45 f4 mov 0xfffffff4(%ebp),%eax
c1070c45: ff 15 60 87 3d c1 call *0xc13d8760
Notice instruction on c1070c39 we have "call *(edx)",
edx was just loaded with the address for the paravirt call.
when we try to replace that to a call to vmi specific function, maybe we
hit a BUG_ON(len < 5) in vmi's patch_internal code, because now the
instruction length is less than 5.
Is there is a way to get GCC to not do such fancy tricks, and instead do
a direct "call 0xc13d8764" ?