[PATCH v2] KVM: VMX: Use try_cmpxchg64 in pi_try_set_control

[PATCH v2] KVM: VMX: Use try_cmpxchg64 in pi_try_set_control

[Uros Bizjak]

Use try_cmpxchg64 instead of cmpxchg64 (*ptr, old, new) != old
in pi_try_set_control.  cmpxchg returns success in ZF flag, so this
change saves a compare after cmpxchg (and related move instruction
in front of cmpxchg):

  b9:   88 44 24 60             mov    %al,0x60(%rsp)
  bd:   48 89 c8                mov    %rcx,%rax
  c0:   c6 44 24 62 f2          movb   $0xf2,0x62(%rsp)
  c5:   48 8b 74 24 60          mov    0x60(%rsp),%rsi
  ca:   f0 49 0f b1 34 24       lock cmpxchg %rsi,(%r12)
  d0:   48 39 c1                cmp    %rax,%rcx
  d3:   75 cf                   jne    a4 <vmx_vcpu_pi_load+0xa4>

patched:

  c1:   88 54 24 60             mov    %dl,0x60(%rsp)
  c5:   c6 44 24 62 f2          movb   $0xf2,0x62(%rsp)
  ca:   48 8b 54 24 60          mov    0x60(%rsp),%rdx
  cf:   f0 48 0f b1 13          lock cmpxchg %rdx,(%rbx)
  d4:   75 d5                   jne    ab <vmx_vcpu_pi_load+0xab>

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Wanpeng Li <wanpengli@tencent.com>
Cc: Jim Mattson <jmattson@google.com>
Cc: Joerg Roedel <joro@8bytes.org>
---
v2:
- use address of local variable &old

The patch is agains tip tree.
---
 arch/x86/kvm/vmx/posted_intr.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/posted_intr.c b/arch/x86/kvm/vmx/posted_intr.c
index 3834bb30ce54..259315ea5e44 100644
--- a/arch/x86/kvm/vmx/posted_intr.c
+++ b/arch/x86/kvm/vmx/posted_intr.c
@@ -42,7 +42,7 @@ static int pi_try_set_control(struct pi_desc *pi_desc, u64 old, u64 new)
 	 * update must be retried with a fresh snapshot an ON change causes
 	 * the cmpxchg to fail.
 	 */
-	if (cmpxchg64(&pi_desc->control, old, new) != old)
+	if (!try_cmpxchg64(&pi_desc->control, &old, new))
 		return -EBUSY;
 
 	return 0;
-- 
2.35.3

Re: [PATCH v2] KVM: VMX: Use try_cmpxchg64 in pi_try_set_control

[kernel test robot]

Hi Uros,

Thank you for the patch! Yet something to improve:

[auto build test ERROR on kvm/master]
[also build test ERROR on mst-vhost/linux-next linux/master linus/master v5.18-rc7]
[If your patch is applied to the wrong git tree, kindly drop us a note.
And when submitting patch, we suggest to use '--base' as documented in
https://git-scm.com/docs/git-format-patch]

url:    https://github.com/intel-lab-lkp/linux/commits/Uros-Bizjak/KVM-VMX-Use-try_cmpxchg64-in-pi_try_set_control/20220520-223925
base:   https://git.kernel.org/pub/scm/virt/kvm/kvm.git master
config: x86_64-allyesconfig (https://download.01.org/0day-ci/archive/20220521/202205211837.X2KKIXbP-lkp@intel.com/config)
compiler: gcc-11 (Debian 11.3.0-1) 11.3.0
reproduce (this is a W=1 build):
        # https://github.com/intel-lab-lkp/linux/commit/84128ccce45cc3ece9b9f8d4df8afa81651547c9
        git remote add linux-review https://github.com/intel-lab-lkp/linux
        git fetch --no-tags linux-review Uros-Bizjak/KVM-VMX-Use-try_cmpxchg64-in-pi_try_set_control/20220520-223925
        git checkout 84128ccce45cc3ece9b9f8d4df8afa81651547c9
        # save the config file
        mkdir build_dir && cp config build_dir/.config
        make W=1 O=build_dir ARCH=x86_64 SHELL=/bin/bash

If you fix the issue, kindly add following tag where applicable
Reported-by: kernel test robot <lkp@intel.com>

All errors (new ones prefixed by >>):

   arch/x86/kvm/vmx/posted_intr.c: In function 'pi_try_set_control':
>> arch/x86/kvm/vmx/posted_intr.c:45:14: error: implicit declaration of function 'try_cmpxchg64'; did you mean 'try_cmpxchg'? [-Werror=implicit-function-declaration]
      45 |         if (!try_cmpxchg64(&pi_desc->control, &old, new))
         |              ^~~~~~~~~~~~~
         |              try_cmpxchg
   cc1: some warnings being treated as errors


vim +45 arch/x86/kvm/vmx/posted_intr.c

    36	
    37	static int pi_try_set_control(struct pi_desc *pi_desc, u64 old, u64 new)
    38	{
    39		/*
    40		 * PID.ON can be set at any time by a different vCPU or by hardware,
    41		 * e.g. a device.  PID.control must be written atomically, and the
    42		 * update must be retried with a fresh snapshot an ON change causes
    43		 * the cmpxchg to fail.
    44		 */
  > 45		if (!try_cmpxchg64(&pi_desc->control, &old, new))
    46			return -EBUSY;
    47	
    48		return 0;
    49	}
    50	

-- 
0-DAY CI Kernel Test Service
https://01.org/lkp

Re: [PATCH v2] KVM: VMX: Use try_cmpxchg64 in pi_try_set_control

[Paolo Bonzini]

Queued, thanks.

Paolo