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Subject: Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack
To: Joerg Roedel <joro@8bytes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>
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From: Andrew Cooper <andrew.cooper3@citrix.com>
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Date: Wed, 17 Jan 2018 14:04:22 +0000
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On 17/01/18 09:02, Joerg Roedel wrote:
> Hi Boris,
>
> thanks for testing this :)
>
> On Tue, Jan 16, 2018 at 09:47:06PM -0500, Boris Ostrovsky wrote:
>> On 01/16/2018 11:36 AM, Joerg Roedel wrote:
>>> +.macro SWITCH_TO_KERNEL_STACK nr_regs=0 check_user=0
>>
>> This (and next patch's SWITCH_TO_ENTRY_STACK) need X86_FEATURE_PTI check.
>>
>> With those macros fixed I was able to boot 32-bit Xen PV guest.
> Hmm, on bare metal the stack switch happens regardless of the
> X86_FEATURE_PTI feature being set, because we always program tss.sp0
> with the systenter stack. How is the kernel entry stack setup on xen-pv?
> I think something is missing there instead.

There is one single stack registered with Xen, on which you get a normal
exception frame in all cases, even via the registered (virtual)
syscall/sysenter/failsafe handlers.

~Andrew