linux-kernel - Re: [Xen-devel] [PATCH] xen/x86: Adjust stack pointer in xen

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Date:	Tue, 17 Nov 2015 11:36:53 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Andrew Cooper <andrew.cooper3@...rix.com>
Cc:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	xen-devel <xen-devel@...ts.xen.org>,
	Borislav Petkov <bp@...en8.de>,
	David Vrabel <david.vrabel@...rix.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

On Tue, Nov 17, 2015 at 11:29 AM, Andrew Cooper
<andrew.cooper3@...rix.com> wrote:
> On 17/11/15 19:16, Andy Lutomirski wrote:
>> On Tue, Nov 17, 2015 at 11:12 AM, Andrew Cooper
>> <andrew.cooper3@...rix.com> wrote:
>>> On 17/11/15 18:49, Andy Lutomirski wrote:
>>>> On Nov 17, 2015 6:40 AM, "Boris Ostrovsky" <boris.ostrovsky@...cle.com> wrote:
>>>>> On 11/16/2015 04:55 PM, H. Peter Anvin wrote:
>>>>>> On 11/16/15 12:22, Borislav Petkov wrote:
>>>>>>> Huh, so what's wrong with a jump:
>>>>>>>
>>>>>>>         jmp 1f
>>>>>>>         swapgs
>>>>>>>         1:
>>>>>>>
>>>>>> What is the point of that jump?
>>>>>>
>>>>>>>> If it would make you feel better, it could be X86_BUG_XENPV :-p
>>>>>>> That doesn't matter - I just don't want to open the flood gates on
>>>>>>> pseudo feature bits.
>>>>>>>
>>>>>>> hpa, what do you think?
>>>>>> Pseudo feature bits are fine, we already have plenty of them.  They make
>>>>>> sense as they let us reuse a lot of infrastructure.
>>>>>
>>>>> So how about something like this? And then I think we can remove usergs_sysret32 and irq_enable_sysexit pv ops completely as noone will use them (lguest doesn't set them)
>>>>>
>>>> Looks good to me.  Does Xen have any sysexit/sysret32 equivalent to
>>>> return to 32-bit user mode?  If so, it could be worth trying to wire
>>>> it up by patching the jz instead of the test instruction.
>>> From the guests point of view, there is only hypercall_iret.
>> Doesn't hypercall_iret have flags that ask for different behavior,
>> though?  (VG_syscall or whatever for the 64-bit case?)
>
> The one and only flag is VGCF_in_syscall
>
> Xen has its own logic for choosing between sysretq/sysretl if
> VGCF_in_syscall, but will end up on an iret path in all other
> circumstances.

In that case, a nicer version of this patch could preserve the new
sysret-or-iret decision (on 64-bit kernels in the compat path) and use
it to set VGCF_in_syscall.  This might work considerably better now
than it ever would have, since Linux now tries to use sysret32 on
*all* 64-bit CPUs, and the way it's structured for compat is just a
flag (the testl %eax,%eax thing) that indicates that sysret32 is okay.

Anyway, that can be a followup patch.

--Andy
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