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Subject: Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack
To: Andrew Cooper <andrew.cooper3@citrix.com>,
        Joerg Roedel <joro@8bytes.org>
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From: Boris Ostrovsky <boris.ostrovsky@oracle.com>
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Date: Wed, 17 Jan 2018 10:22:24 -0500
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On 01/17/2018 09:04 AM, Andrew Cooper wrote:
> 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.

And so the check should be at least against X86_FEATURE_XENPV, not
necessarily X86_FEATURE_PTI.

But I guess you can still check against X86_FEATURE_PTI since without it
there is not much reason to switch stacks?

-boris