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

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Message-ID: <CALCETrVAEKrPiMX9O43pqkEC=xqc2qJ=VBNsV5iMMvW09W07hQ@mail.gmail.com>
Date:	Fri, 13 Nov 2015 15:26:04 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
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 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.

--Andy
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