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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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.

> 
> 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.

> 
> thanks,
> Mukesh
> 
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