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[PATCH 1/2] KVM: x86: introduce do_shl32_div32

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This is similar to the existing div_frac function, but it returns the
remainder too.  Unlike div_frac, it can be used to implement long
division, e.g. (a << 64) / b for 32-bit a and b.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 arch/x86/kvm/x86.c | 10 ++--------
 arch/x86/kvm/x86.h | 15 +++++++++++++++
 2 files changed, 17 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4244c2baf57d..5b937fdebc66 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1196,14 +1196,8 @@ static void kvm_write_wall_clock(struct kvm *kvm, gpa_t wall_clock)
 
 static uint32_t div_frac(uint32_t dividend, uint32_t divisor)
 {
-	uint32_t quotient, remainder;
-
-	/* Don't try to replace with do_div(), this one calculates
-	 * "(dividend << 32) / divisor" */
-	__asm__ ( "divl %4"
-		  : "=a" (quotient), "=d" (remainder)
-		  : "0" (0), "1" (dividend), "r" (divisor) );
-	return quotient;
+	do_shl32_div32(dividend, divisor);
+	return dividend;
 }
 
 static void kvm_get_time_scale(uint32_t scaled_khz, uint32_t base_khz,
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index f2afa5fe48a6..34f416427143 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -192,4 +192,19 @@ extern unsigned int min_timer_period_us;
 extern unsigned int lapic_timer_advance_ns;
 
 extern struct static_key kvm_no_apic_vcpu;
+
+/* Same "calling convention" as do_div:
+ * - divide (n << 32) by base
+ * - put result in n
+ * - return remainder
+ */
+#define do_shl32_div32(n, base)					\
+	({							\
+	    u32 __quot, __rem;					\
+	    asm("divl %2" : "=a" (__quot), "=d" (__rem)		\
+			: "rm" (base), "0" (0), "1" ((u32) n));	\
+	    n = __quot;						\
+	    __rem;						\
+	 })
+
 #endif
-- 
1.8.3.1

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[RFC PATCH 0/2] Hyper-V TSC page support. Paolo Bonzini <pbonzini@redhat.com> - 2016-01-28 15:10 +0100
  [PATCH 2/2] kvm/x86: Hyper-V tsc page setup Paolo Bonzini <pbonzini@redhat.com> - 2016-01-28 15:10 +0100
  [PATCH 1/2] KVM: x86: introduce do_shl32_div32 Paolo Bonzini <pbonzini@redhat.com> - 2016-01-28 15:10 +0100

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