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SubjectRe: [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function
On Fri, Feb 10, 2017 at 10:43:09AM -0500, Waiman Long wrote:
> It was found when running fio sequential write test with a XFS ramdisk
> on a VM running on a 2-socket x86-64 system, the %CPU times as reported
> by perf were as follows:
>
> 69.75% 0.59% fio [k] down_write
> 69.15% 0.01% fio [k] call_rwsem_down_write_failed
> 67.12% 1.12% fio [k] rwsem_down_write_failed
> 63.48% 52.77% fio [k] osq_lock
> 9.46% 7.88% fio [k] __raw_callee_save___kvm_vcpu_is_preempt
> 3.93% 3.93% fio [k] __kvm_vcpu_is_preempted
>

Thinking about this again, wouldn't something like the below also work?


diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 099fcba4981d..6aa33702c15c 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 +598,31 @@ __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);
+
+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
+"push %rdi;"
+"push %rdx;"
+"movslq %edi, %rdi;"
+"movq $steal_time+16, %rax;"
+"movq __per_cpu_offset(,%rdi,8), %rdx;"
+"cmpb $0, (%rdx,%rax);"
+"setne %al;"
+"pop %rdx;"
+"pop %rdi;"
+FRAME_END
+"ret;"
+".popsection");
+
+#endif
+
/*
* Setup pv_lock_ops to exploit KVM_FEATURE_PV_UNHALT if present.
*/
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