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Re: [Xen-devel] Crash with paravirt-ops kernel

To: Ian Campbell <Ian.Campbell@xxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] Crash with paravirt-ops kernel
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 23 Nov 2009 17:13:15 +0000
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Bastian Blank <bastian@xxxxxxxxxxxx>, "544145@xxxxxxxxxxxxxxx" <544145@xxxxxxxxxxxxxxx>
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On 23/11/2009 16:44, "Ian Campbell" <Ian.Campbell@xxxxxxxxxx> wrote:

>> But this is not just the return-to-user-space path you're changing, but
>> also the hypercall one. You certainly don't want an iret in that case.
> Don't the hypercalls already always go via iret?
> -        testw $TRAP_syscall,4(%rsp)
> -        jz    iret_exit_to_guest
> IOW if TRAP_syscall is not set (i.e. this is a hypercall not a syscall)
> then exit via iret.

I think not -- here TRAP_syscall means 'entered Xen via SYSCALL
instruction', not 'entered to do a syscall'. TRAP_syscall should be set
regardless of whether the SYSCALL instruction was executed by guest userland
or guest kernel.

 -- Keir

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