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

On 10/27/2014 02:04 PM, Peter Zijlstra wrote:
On Mon, Oct 27, 2014 at 01:38:20PM -0400, Waiman Long wrote:
On 10/24/2014 04:54 AM, Peter Zijlstra wrote:
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:


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");

+DEF_NATIVE(pv_lock_ops, queue_unlock, "movb $0, (%rdi)");
  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);
+               PATCH_SITE(pv_lock_ops, queue_unlock);

                 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.
My concern is that spin_unlock() can be called in many places, including
loadable kernel modules. Can the paravirt_patch_ident_32() function able to
patch all of them in reasonable time? How about a kernel module loaded later
at run time?
modules should be fine, see arch/x86/kernel/module.c:module_finalize()
->  apply_paravirt().

Also note that the 'default' text is an indirect call into the paravirt
ops table which routes to the 'right' function, so even if the text
patching would be 'late' calls would 'work' as expected, just slower.

Thanks for letting me know about that. I have this concern because your patch didn't change the current configuration of disabling unlock inlining when paravirt_spinlock is enabled. With that, I think it is worthwhile to reduce the performance delta between the PV and non-PV kernel on bare metal.


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