Re: [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

From: Boris Ostrovsky
Date: Fri Nov 13 2015 - 20:24:03 EST

On 11/13/2015 06:26 PM, Andy Lutomirski wrote:
On Fri, Nov 13, 2015 at 3:18 PM, Boris Ostrovsky
<boris.ostrovsky@xxxxxxxxxx> 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 */

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.

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