On Tue, Feb 14, 2017 at 09:46:17AM -0500, Waiman Long wrote:
> On 02/14/2017 04:39 AM, Peter Zijlstra wrote:
> > On Mon, Feb 13, 2017 at 05:34:01PM -0500, Waiman Long wrote:
> >> It is the address of &steal_time that will exceed the 32-bit limit.
> > That seems extremely unlikely. That would mean we have more than 4G
> > worth of per-cpu variables declared in the kernel.
>
> I have some doubt about if the compiler is able to properly use
> RIP-relative addressing for this.
Its not RIP relative, &steal_time lives in the .data..percpu section and
is absolute in that.
> Anyway, it seems like constraints
> aren't allowed for asm() when not in the function context, at least for
> the the compiler that I am using (4.8.5). So it is a moot point.
Well kvm_steal_time is (host/guest) ABI anyway, so the offset is fixed
and hard-coding it isn't a problem.
$ readelf -s defconfig-build/vmlinux | grep steal_time
100843: 0000000000017ac0 64 OBJECT WEAK DEFAULT 35 steal_time
$ objdump -dr defconfig-build/vmlinux | awk '/[<][^>]*[>]:/ { o=0 } /[<]__raw_callee_save___kvm_vcpu_is_preempted[>]:/ {o=1} { if (o) print $0 }'
ffffffff810b4480 <__raw_callee_save___kvm_vcpu_is_preempted>:
ffffffff810b4480: 55 push %rbp
ffffffff810b4481: 48 89 e5 mov %rsp,%rbp
ffffffff810b4484: 48 8b 04 fd 00 94 46 mov -0x7db96c00(,%rdi,8),%rax
ffffffff810b448b: 82
ffffffff810b4488: R_X86_64_32S __per_cpu_offset
ffffffff810b448c: 80 b8 d0 7a 01 00 00 cmpb $0x0,0x17ad0(%rax)
ffffffff810b448e: R_X86_64_32S steal_time+0x10
ffffffff810b4493: 0f 95 c0 setne %al
ffffffff810b4496: 5d pop %rbp
ffffffff810b4497: c3 retq
And as you'll note, the displacement is correct and 'small'.
The below relies on the 'extra' cast in PVOP_CALL_ARG1() to extend the
argument to 64bit on the call side of things.
---
arch/x86/kernel/kvm.c | 21 +++++++++++++++++++++
1 file changed, 21 insertions(+)
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 099fcba..2c854b8 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -589,6 +589,7 @@ static void kvm_wait(u8 *ptr, u8 val)
local_irq_restore(flags);
}
+#ifdef CONFIG_X86_32
__visible bool __kvm_vcpu_is_preempted(int cpu)
{
struct kvm_steal_time *src = &per_cpu(steal_time, cpu);
@@ -597,6 +598,26 @@ __visible bool __kvm_vcpu_is_preempted(int cpu)
}
PV_CALLEE_SAVE_REGS_THUNK(__kvm_vcpu_is_preempted);
+#else
+
+extern bool __raw_callee_save___kvm_vcpu_is_preempted(int cpu);
+
+asm(
+".pushsection .text;"
+".global __raw_callee_save___kvm_vcpu_is_preempted;"
+".type __raw_callee_save___kvm_vcpu_is_preempted, @function;"
+"__raw_callee_save___kvm_vcpu_is_preempted:"
+FRAME_BEGIN
+"movq __per_cpu_offset(,%rdi,8), %rax;"
+"cmpb $0, 16+steal_time(%rax);"
+"setne %al;"
+FRAME_END
+"ret;"
+".popsection"
+);
+
+#endif
+
/*
* Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
*/
