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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

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 */
> 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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