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

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Date:	Tue, 17 Nov 2015 09:40:23 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	"H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>,
	Andy Lutomirski <luto@...capital.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	xen-devel <xen-devel@...ts.xen.org>,
	David Vrabel <david.vrabel@...rix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH] xen/x86: Adjust stack pointer in xen_sysexit

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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