On 10/15/2012 11:38 PM, HATAYAMA Daisuke wrote: > > Thanks for pointing out this. And I've recalled my investigation in > the past now. So I want to stop retrying your patch v9 now. This NMI > method is definitely not applicable to 2nd kernel without any change. > > Your NMI method assumes BSP thread is halting in play dead loop. But > on the 2nd kernel, BSP is halting in the 1st kernel (or possibly in a > fatail system error). Even if throwing NMI to BSP, it goes back to the > 1st kernel soon again. I at least confirmed NMI handler is executed in > this case. > > Also, throwing NMI changes stack in the 1st kernel, which is > unpermissible from kdump's perspective. But x86_64 uses Interrupt > Stack Table (IST), in which stack switching is not performed. So 2nd > kernel's stack is used at least on x86_64. > > To sum up, to apply NMI method in the 2nd kernel, I think it necessary > to modify contexts pushed on the stack so execution goes to the 2nd > kernel's start_secondary() while initializing its state > appropreately. > > Also I think it necessary to discuss whether this NMI method is > reliable enough for kdump use. > I think it's pretty clear it is *not*. NMI or monitor would either have to rely on context set up by the first kernel, which simply isn't safe. Out of those two options, a monitor would actually be safer, since it can be self-contained in a completely different way. However, it seems that running on N-1 CPUs in kdump is perfectly acceptable. -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
