Linux Virtualization - Feb 2017 - [PATCH v2] x86/paravirt: Don't make vcpu_is_preempted() a callee-save function

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.

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. 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.

Cheers,
Longman

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.
  */

On 14/02/17 14:46, 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. 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.
You can work the issue of not having parameters in a plain asm()
statement by using an asm-offset, stringizing it, and have C put the
string fragments back together.

"cmpb $0, " STR(STEAL_TIME_preempted) "(%rax);"

~Andrew