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SubjectRe: [PATCH] xen/x86: Adjust stack pointer in xen_sysexit
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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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