Message-ID: <4421E3C4.2060808@vmware.com>
Date: Wed, 22 Mar 2006 15:54:44 -0800
From: Zachary Amsden <zach@vmware.com>
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To: Andi Kleen <ak@suse.de>
Cc: virtualization@lists.osdl.org, Linus Torvalds <torvalds@osdl.org>,
       Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
       Xen-devel <xen-devel@lists.xensource.com>,
       Andrew Morton <akpm@osdl.org>, Dan Hecht <dhecht@vmware.com>,
       Dan Arai <arai@vmware.com>, Anne Holler <anne@vmware.com>,
       Pratap Subrahmanyam <pratap@vmware.com>,
       Christopher Li <chrisl@vmware.com>, Joshua LeVasseur <jtl@ira.uka.de>,
       Chris Wright <chrisw@osdl.org>, Rik Van Riel <riel@redhat.com>,
       Jyothy Reddy <jreddy@vmware.com>, Jack Lo <jlo@vmware.com>,
       Kip Macy <kmacy@fsmware.com>, Jan Beulich <jbeulich@novell.com>,
       Ky Srinivasan <ksrinivasan@novell.com>,
       Wim Coekaerts <wim.coekaerts@oracle.com>,
       Leendert van Doorn <leendert@watson.ibm.com>
Subject: Re: [RFC, PATCH 1/24] i386 Vmi documentation II
References: <200603131759.k2DHxeep005627@zach-dev.vmware.com> <200603222239.46604.ak@suse.de> <4421D379.3090405@vmware.com> <200603222338.44919.ak@suse.de>
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Andi Kleen wrote:
> On Wednesday 22 March 2006 23:45, Zachary Amsden wrote:
>
>   
>> I propose an entirely different approach - use segmentation. 
>>     
>
> That would require a lot of changes to save/restore the segmentation
> register at kernel entry/exit since there is no swapgs on i386. 
> And will be likely slower there too and also even slow down the 
> VMI-kernel-no-hypervisor.
>   

There are no changes required to the kernel entry / exit paths.  With 
save/restore segment support in the VMI, reserving one segment for the 
hypervisor data area is easy.

I take it back.  There is one required change:

kernel_entry:
     hypervisor_entry_hook
     sti
     .... kernel code

This hypervisor_entry_hook can be a nop on native hardware, and the 
following for Xen:

push %gs
mov CPU_HYPER_SEL, %gs
pop %gs:SAVED_USER_GS

You already have the IRET / SYSEXIT hooks to restore it on the way 
back.  And now you have a segment reserved that allows you to deal with 
16-bit stack segments during the IRET.

> Still might be the best option.
>
> How did that rumoured Xenolinux-over-VMI implementation solve that problem?
>   

!CONFIG_SMP  -- as I believe I saw in the latest Xen patches sent out as 
well?
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