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

To: Jan Beulich <jbeulich@xxxxxxxxxx>, Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] Re: sysenter/syscall support for 32-on-64 guests?
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Thu, 28 Feb 2008 21:22:38 +0000
Cc: Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Ian Campbell <ijc@xxxxxxxxxxxxxx>
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On 28/2/08 17:14, "Jan Beulich" <jbeulich@xxxxxxxxxx> wrote:

>> 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

Ah yes, I must admit I temporarily forgot you can only target 64-bit code
segments in a long-mode IDT.

 -- Keir

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