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[Xen-devel] Re: sysenter/syscall support for 32-on-64 guests?

To: "Jeremy Fitzhardinge" <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: sysenter/syscall support for 32-on-64 guests?
From: "Jan Beulich" <jbeulich@xxxxxxxxxx>
Date: Thu, 28 Feb 2008 17:14:21 +0000
Cc: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Campbell <ijc@xxxxxxxxxxxxxx>
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>>> 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


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