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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 11/16/2015 04:55 PM, H. Peter Anvin wrote:
> On 11/16/15 12:22, Borislav Petkov wrote:
> > Huh, so what's wrong with a jump:
> >
> >         jmp 1f
> >         swapgs
> >         1:
> What is the point of that jump?
>
> > > If it would make you feel better, it could be X86_BUG_XENPV :-p
> > That doesn't matter - I just don't want to open the flood gates on
> > pseudo feature bits.
> >
> > hpa, what do you think?
> Pseudo feature bits are fine, we already have plenty of them.  They make
> sense as they let us reuse a lot of infrastructure.

So how about something like this? And then I think we can remove 
usergs_sysret32 and irq_enable_sysexit pv ops completely as noone will 
use them (lguest doesn't set them)
diff --git a/arch/x86/entry/entry_32.S b/arch/x86/entry/entry_32.S
index 3eb572e..c43df7b 100644
--- 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
+       /* PV guests always use IRET path */
+       ALTERNATIVE "testl %eax, %eax", "jmp .Lsyscall_32_done", 
X86_FEATURE_PV
        jz      .Lsyscall_32_done

 /* Opportunistic SYSEXIT */
diff --git a/arch/x86/entry/entry_64_compat.S 
b/arch/x86/entry/entry_64_compat.S
index c320183..2d1bc82 100644
--- a/arch/x86/entry/entry_64_compat.S
+++ b/arch/x86/entry/entry_64_compat.S
@@ -121,7 +121,7 @@ sysenter_flags_fixed:

        movq    %rsp, %rdi
        call    do_fast_syscall_32
-       testl   %eax, %eax
+       ALTERNATIVE "testl %eax, %eax", "jmp .Lsyscall_32_done", 
X86_FEATURE_PV
        jz      .Lsyscall_32_done
        jmp     sysret32_from_system_call

@@ -200,7 +200,8 @@ ENTRY(entry_SYSCALL_compat)

        movq    %rsp, %rdi
        call    do_fast_syscall_32
-       testl   %eax, %eax
+       /* PV guests always use IRET path */
+       ALTERNATIVE "testl %eax, %eax", "jmp .Lsyscall_32_done", 
X86_FEATURE_PV
        jz      .Lsyscall_32_done

        /* Opportunistic SYSRET */
diff --git a/arch/x86/include/asm/cpufeature.h 
b/arch/x86/include/asm/cpufeature.h
index e4f8010..723327b 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_PV          ( 8*32+16) /* Paravirtual guest */


 /* Intel-defined CPU features, CPUID level 0x00000007:0 (ebx), word 9 */



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