Re: [Xen-devel] [PATCH 1/3] x86/xen: Avoid fast syscall path for Xen PV guests

[Andy Lutomirski <luto@xxxxxxxxxxxxxx>]

On Wed, Nov 18, 2015 at 12:06 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).
>
> Since we end up calling xen_iret from xen_sysexit we don't need to fix
> up the stack and instead follow entry_SYSENTER_32's IRET path directly
> to xen_iret.
>
> We can do the same thing for compat mode even though stack does not need
> to be fixed. This will allow us to drop usergs_sysret32 paravirt op (in
> the subsequent patch)

Looks generally quite nice.  Minor comments below:

> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -308,7 +308,8 @@ sysenter_past_esp:
>
>         movl    %esp, %eax
>         call    do_fast_syscall_32
> -       testl   %eax, %eax
> +       /* XEN PV guests always use IRET path */
> +       ALTERNATIVE "testl %eax, %eax", "xor %eax, %eax", X86_FEATURE_XENPV
>         jz      .Lsyscall_32_done

Could we make this a little less subtle:

ALTERNATIVE "testl %eax, %eax; lz .Lsyscall_32_done", "jmp
.Lsyscasll_32_done", X86_FEATURE_XENPV

Borislav, what do you think?

Ditto for the others.

> diff --git a/arch/x86/include/asm/cpufeature.h 
> b/arch/x86/include/asm/cpufeature.h
> index e4f8010..0e4fe5b 100644
> --- a/arch/x86/include/asm/cpufeature.h
> +++ b/arch/x86/include/asm/cpufeature.h
> @@ -216,6 +216,7 @@
>  #define X86_FEATURE_PAUSEFILTER ( 8*32+13) /* AMD filtered pause intercept */
>  #define X86_FEATURE_PFTHRESHOLD ( 8*32+14) /* AMD pause filter threshold */
>  #define X86_FEATURE_VMMCALL     ( 8*32+15) /* Prefer vmmcall to vmcall */
> +#define X86_FEATURE_XENPV       ( 8*32+16) /* Xen paravirtual guest */
>

This bit is highly magical and I think we need Borislav's ack.

--Andy

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