[Devel] Re: [PATCH 1/1] fill vdso with syscall32_setup_pages if TIF_IA32 on x86_64

[Serge E. Hallyn]

Quoting Oren Laadan (orenl at cs.columbia.edu):
> 
> 
> Serge E. Hallyn wrote:
> >Quoting Oren Laadan (orenl at cs.columbia.edu):
> >>
> >>Serge E. Hallyn wrote:
> >>>Quoting Oren Laadan (orenl at cs.columbia.edu):
> >>>>Serge E. Hallyn wrote:
> >>>>>Quoting Oren Laadan (orenl at cs.columbia.edu):
> >>>>>>Serge E. Hallyn wrote:
> >>>>>>>Quoting Oren Laadan (orenl at cs.columbia.edu):
> >>>>>>>>Cool !
> >>>>>>>>
> >>>>>>>>So what do we have working now for 64 bit kernel (for 32 bit kernel
> >>>>>>>>we know it works...):
> >>>>>>>>
> >>>>>>>>	'restart'	checkpointed
> >>>>>>>>	 program	  program
> >>>>>>>>	----------------------------------------
> >>>>>>>>	  64bit		  64bit		-> works
> >>>>>>>>	  32bit		  32bit		-> works
> >>>>>>>>
> >>>>>>>>	  64bit		  32bit		-> ?????
> >>>s/?????/Rejected/
> >>>
> >>>CKPT_ARCH_ID is of course different for X86_32 than X86_64, so
> >>>we refuse restart in restore_read_header().
> >>>
> >>>-serge
> >>>
> >>lol ... that's actually funny !
> >>
> >>Anyway, in light of the IRC discussions, here are the cases again:
> >>
> >>
> >>original	original	restart		target
> >>program		kernel		program		kernel
> >>--------	---------	--------	--------
> >>64 bit		64 bit		64 bit		64 bit	  [0] works
> >>
> >>32 bit		32 bit		32 bit		32 bit	  [0] works
> >>32 bit		64 bit		32 bit		64 bit	  [0] works
> >>
> >>32 bit		32 bit		32 bit		64 bit	  [1]
> >>32 bit		64 bit		32 bit		32 bit	  [1]
> >>
> >>32 bit		any		64 bit		64 bit	  [2]
> >>64 bit		64 bit		32 bit		64 bit	  [2]
> >>
> >>[0] The first 3 cases are "homogeneous", with conditions equal at
> >>checkpoint and restart. AFAIK, they work.
> >>
> >>[1] The next two cases consider 32 bit program, and vary only the
> >>environment - the kernel may change from 32 to 64 or back. We want
> >>them to work.
> >>
> >>IIUC, your comment above means that they don't work because the
> >>CKPT_ARCH_ID is a mismatch. The fix should be trivial - either
> >>make 'restart' modify it, or make the kernel tolerate it.
> >
> >Well, you'd think so, but we also check for uts->machine, and want
> >to eventually check for kernel config, both of which are obviously
> >different.
> 
> Then we'll have to take that in account when we get to also
> check those other fields.
> 
> >
> >After I comment out the obvious offending checks, it still fails to
> >restart from x8632->x86-64.  I can spend some time next week figuring
> >out what we're not quite doing right as there shouldn't be a
> >problem really.  But do we definately want to go out of our way to try
> > and mask out the differences in this case, while trying to detect
> >cpu differences between two x86-32's for instance?
> 
> I agree, there shouldn't be a problem really, and I expect this to
> be a very useful feature for migration/fault-tolerance.

May be, but then perhaps this is the first case where we should be
using a userspace checkpoing image rewriter to help us out.  Otherwise
we'll need to hardcode in the kernel that a task which was
checkpointed on X86_32 should, on x86_64, have TIF_IA32 added to
the thread_flags but may be restarted;  etc.  Should be doable, but
kind of ugly...

-serge
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