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

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Message-ID: <56468D24.8030801@oracle.com>
Date:	Fri, 13 Nov 2015 20:23:48 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	David Vrabel <david.vrabel@...rix.com>,
	xen-devel <xen-devel@...ts.xen.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Andrew Lutomirski <luto@...nel.org>
Subject: Re: [PATCH] xen/x86: Adjust stack pointer in xen_sysexit



On 11/13/2015 06:26 PM, Andy Lutomirski wrote:
> On Fri, Nov 13, 2015 at 3:18 PM, Boris Ostrovsky
> <boris.ostrovsky@...cle.com> wrote:
>> After 32-bit syscall rewrite, and specifically after commit 5f310f739b4c
>> ("x86/entry/32: Re-implement SYSENTER using the new C path"), the stack
>> frame that is passed to xen_sysexit is no longer a "standard" one (i.e.
>> it's not pt_regs).
>>
>> We need to adjust it so that subsequent xen_iret can use it.
> I'm wondering if this should be more straightforward:
>
>          movq    %rsp, %rdi
>          call    do_fast_syscall_32
>          testl   %eax, %eax
>          jz      .Lsyscall_32_done
>
>          /* Opportunistic SYSRET */
> sysret32_from_system_call:
>          XEN_DO_SYSRET32
>
> where XEN_DO_SYSRET32 is a simple pv op that, on Xen, jumps to a
> variant of Xen's iret path that knows that the fast path is okay.


This patch is for 32-bit kernel. I actually haven't looked at compat 
code (probably because our tests don't try that), I need to do that too.

As for XEN_DO_SYSRET32 --- we'd presumably need to have a nop for 
baremetal otherwise current paravirt op will use native_usergs_sysret32 
(for compat code). Which means a new pv_op, I think.

-boris
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