Re: [Xen-devel] [HYBRID]: XEN_EMULATE_PREFIX in user process

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-06-29 at 02:26 +0100, Mukesh Rathor wrote:
> On Thu, 28 Jun 2012 21:09:03 -0400
> Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> 
> > On Thu, Jun 28, 2012 at 06:00:07PM -0700, Mukesh Rathor wrote:
> > > Hi,
> > > 
> > > Booting latest upstream pv linux as dom0, I noticed dm_mapper, user
> > > level process, is running with XEN_EMULATE_PREFIX cpuid. I am
> > > wondering if the prefix is statically put there, or how its put
> > > there?
> > 
> > That is impressive. It should be only be in the kernel via the pvops
> > cpuid call which does this:
> > 
> >  252         asm(XEN_EMULATE_PREFIX "cpuid"
> >  253                 : "=a" (*ax),
> >  254                   "=b" (*bx),
> >  255                   "=c" (*cx),
> >  256                   "=d" (*dx)
> >  257                 : "0" (*ax), "2" (*cx));
> >  258 
> > 
> > 
> > > 
> > > For hybrid, the hvm container traps cpuid, so we don't need to
> > > prefix cpuid. Less code if I don't have to worry about trapping the
> > > invalid op. There seems places where the cpuid is run without the
> > > xen prefix in user mode.
> > 
> > Right, you can make the .cpuid pvops call point to the native.
> 
> Yup. In the kernel I do that already.
> 
> > > 
> > > BTW, for cpuid vmexit, if kernel mode I call pv_cpuid, if user
> > > mode, I just return cpuid values, as that's what would happen
> > > without the HVM container.
> > 
> > You mean in PV mode in user-space you would get the unfiltered cpuid
> > value? Yes, that is true. Which is why I am surprised that dm_mapper
> > (are you sure it is not a kernel thread?) is doing that.
> 
> User mode:
> 
> RIP: 0x0000000000400692 CS: 0x0033

CS 0x033 could still be kernel mode on regular PV 64 bit, but I suppose
not on hybrid?

It's unlikely but not impossible for a userspace developer to have done
some "Xen magic" and used the prefix in userspace.

What version of dm_mapper do you have? I checked the version in Debian
Sid and it doesn't do this (at least not directly).

Are you able to run the dm_mapper process under gdb and inspect it to
find the prefix?

Ian.

> 
> [0]xkdb> dd 0x0000000000400680 32 0
> 0000000000400680:  89f88953e5894855 db8500000000b9d3
> 0000000000400690:  0f6e65780b0f0574 4e89045e890689a2
> 
> thanks,
> Mukesh
> 
> 
> 
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