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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Tue, 17 Jul 2018 13:06:11 -0700
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Subject: Re: [PATCH 10/39] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack
To:     Joerg Roedel <jroedel@suse.de>
Cc:     Andy Lutomirski <luto@kernel.org>, Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>,
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        Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Brian Gerst <brgerst@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Eduardo Valentin <eduval@amazon.com>,
        Greg KH <gregkh@linuxfoundation.org>,
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        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, Pavel Machek <pavel@ucw.cz>,
        "David H . Gutteridge" <dhgutteridge@sympatico.ca>
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On Tue, Jul 17, 2018 at 12:15 AM, Joerg Roedel <jroedel@suse.de> wrote:
> On Sat, Jul 14, 2018 at 07:36:47AM -0700, Andy Lutomirski wrote:
>> But I=E2=80=99m still unconvinced. If any code executed with IRQs enable=
d on
>> the entry stack, then that code is terminally buggy. If you=E2=80=99re
>> executing with IRQs off, you=E2=80=99re not going to get migrated.  64-b=
it
>> kernels run on percpu stacks all the time, and it=E2=80=99s not a proble=
m.
>
> The code switches to the kernel-stack and kernel-cr3 and just remembers
> where it came from (to handle the entry-from-kernel with entry-stack
> and/or user-cr3 case). IRQs are disabled in the entry-code path. But
> ultimately it calls into C code to handle the exception. And there IRQs
> might get enabled again.
>
>> IRET errors are genuinely special and, if they=E2=80=99re causing a prob=
lem
>> for you, we should fix them the same way we deal with them on x86_64.
>
> Right, IRET is handled differently and doesn't need this patch. But the
> segment-writing exceptions do.
>
> If you insist on it I can try to implement the assumption that we don't
> get preempted in this code-path. That will safe us some cycles for
> copying stack contents in this unlikely slow-path. But we definitly need
> to handle the entry-from-kernel with entry-stack and/or user-cr3 case
> correctly and make a switch to kernel-stack/cr3 because we are going to
> call into C-code.
>
>

Yes, we obviously need to restore the correct cr3.  But I really don't
like the code that rewrites the stack frame that we're about to IRET
to, especially when it doesn't seem to serve a purpose.  I'd much
rather the code just get its CR3 right and do the IRET and trust that
the frame it's returning to is still there.