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xen-devel
[Xen-devel] Re: sysenter/syscall support for 32-on-64 guests?
>>> Jeremy Fitzhardinge <jeremy@xxxxxxxx> 28.02.08 17:49 >>>
>I just spent quite a while working out why pvops-Xen kernels weren't
>booting for me in a 64-bit hypervisor. It turned out to be because
>32-on-64 supports SEP even though 32-on-32 doesn't, so the guest kernel
>was trying to use it without having set up the Xen side, resulting in
>syscalls jumping to 0:0, generating a mysterious GP. Once I worked that
>out, the fix was simple, of course.
>
>Anyway, I wonder if you had a specific reason for allowing this, other
>than "because we can"? It seems to me that we should try to minimize
>the number of differences between 32-on-32 and 32-on-64 as much as possible.
Since int80 cannot be directly passed to the guest (as in 32-on-32), it
seemed reasonable to cut on the overhead of syscalls by at least
allowing this (and syscall could also be made work now that the vdso
stuff is unified in 2.6.25) - for obvious reasons it still goes through the
hypervisor, but gets there faster
Jan
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