Re: [Xen-devel] [PATCH, fixed] linux/x86: use sysenter/syscall for 32-bit apps on 64-bit Xen

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 24/10/07 15:28, "Andi Kleen" <ak@xxxxxxx> wrote:

> Perhaps pointing out the obvious; but your comment seems a little strange
> or I am misunderstanding something.
> 
> Of course does a 32bit kernel native kernel use  SYSENTER.
> 
> The only CPU with SYSCALL and no SYSENTER was AMD K6, but there SYSCALL
> was slightly different and unusable (IIRC).
> 
> So it always uses SYSENTER because it SYSCALL doesn't make sense.
> 
> But for compat tasks running on a 64bit kernel obviously SYSCALL
> has to be used and that is what the native kernel does transparently
> using the  32bit vsyscall page.

The issue here is 32-bit application on 32-bit kernel on 64-bit hypervisor.
In this configuration we cannot use SYSENTER on AMD CPUs (as we could with a
32-bit kernel on native) as we are not in legacy mode. But we could use
SYSCALL, except that the code is not there in i386 Linux (for the reason you
state above) and Jan has not seen fit to add it.

 -- Keir



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