linux-kernel - Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack

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Date:   Wed, 17 Jan 2018 14:04:22 +0000
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Joerg Roedel <joro@...tes.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, <x86@...nel.org>,
        <linux-kernel@...r.kernel.org>, <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>, <aliguori@...zon.com>,
        <daniel.gruss@...k.tugraz.at>, <hughd@...gle.com>,
        <keescook@...gle.com>, Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, <jroedel@...e.de>
Subject: Re: [PATCH 02/16] x86/entry/32: Enter the kernel via trampoline stack

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