Re: [Xen-devel] [PATCH v12 09/11] pvqspinlock, x86: Add para-virtualization support

[Peter Zijlstra <peterz@xxxxxxxxxxxxx>]

On Thu, Oct 16, 2014 at 02:10:38PM -0400, Waiman Long wrote:

> Since enabling paravirt spinlock will disable unlock function inlining,
> a jump label can be added to the unlock function without adding patch
> sites all over the kernel.

But you don't have to. My patches allowed for the inline to remain,
again reducing the overhead of enabling PV spinlocks while running on a
real machine.

Look at: 

  http://lkml.kernel.org/r/20140615130154.213923590@xxxxxxxxx

In particular this hunk:

Index: linux-2.6/arch/x86/kernel/paravirt_patch_64.c
===================================================================
--- linux-2.6.orig/arch/x86/kernel/paravirt_patch_64.c
+++ linux-2.6/arch/x86/kernel/paravirt_patch_64.c
@@ -22,6 +22,10 @@ DEF_NATIVE(pv_cpu_ops, swapgs, "swapgs")
 DEF_NATIVE(, mov32, "mov %edi, %eax");
 DEF_NATIVE(, mov64, "mov %rdi, %rax");

+#if defined(CONFIG_PARAVIRT_SPINLOCKS) && defined(CONFIG_QUEUE_SPINLOCK)
+DEF_NATIVE(pv_lock_ops, queue_unlock, "movb $0, (%rdi)");
+#endif
+ 
 unsigned paravirt_patch_ident_32(void *insnbuf, unsigned len)
 {
        return paravirt_patch_insns(insnbuf, len,
@@ -61,6 +65,9 @@ unsigned native_patch(u8 type, u16 clobb
                PATCH_SITE(pv_cpu_ops, clts);
                PATCH_SITE(pv_mmu_ops, flush_tlb_single);
                PATCH_SITE(pv_cpu_ops, wbinvd);
+#if defined(CONFIG_PARAVIRT_SPINLOCKS) && defined(CONFIG_QUEUE_SPINLOCK)
+               PATCH_SITE(pv_lock_ops, queue_unlock);
+#endif

        patch_site:
                ret = paravirt_patch_insns(ibuf, len, start, end);


That makes sure to overwrite the callee-saved call to the
pv_lock_ops::queue_unlock with the immediate asm "movb $0, (%rdi)".


Therefore you can retain the inlined unlock with hardly (there might be
some NOP padding) any overhead at all. On PV it reverts to a callee
saved function call.

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