Use 'regs' as a contiguous linear bitmap in  apic_{set|
clear|test}_vector() while doing bitwise operations.
This makes the code simpler by eliminating the need to
determine the offset of the 32-bit register and the vector
bit location within that register prior to performing
bitwise operations.

This change results in slight increase in generated code
size for gcc-14.2.

- Without change

apic_set_vector:
89 f8             mov    %edi,%eax
83 e7 1f          and    $0x1f,%edi
c1 e8 05          shr    $0x5,%eax
c1 e0 04          shl    $0x4,%eax
48 01 c6          add    %rax,%rsi
f0 48 0f ab 3e    lock bts %rdi,(%rsi)
c3                ret

- With change

apic_set_vector:

89 f8             mov    %edi,%eax
c1 e8 05          shr    $0x5,%eax
8d 04 40          lea    (%rax,%rax,2),%eax
c1 e0 05          shl    $0x5,%eax
01 f8             add    %edi,%eax
89 c0             mov    %eax,%eax
f0 48 0f ab 3e    lock bts %rax,(%rsi)
c3                ret

But, lapic.o text size (bytes) decreases with this change:

Obj        Old-size      New-size

lapic.o    28832         28768

Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
---
Changes since v6:

 - Converted assmebly to AT&T format.
 - Added information about overall kvm.ko text size change.

 arch/x86/include/asm/apic.h | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h
index b7cbe9ba363e..f91d23757375 100644
--- a/arch/x86/include/asm/apic.h
+++ b/arch/x86/include/asm/apic.h
@@ -564,19 +564,28 @@ static __always_inline void apic_set_reg64(void *regs, int reg, u64 val)
 	ap->regs64[reg / 8] = val;
 }
 
+static inline unsigned int get_vec_bit(unsigned int vec)
+{
+	/*
+	 * The registers are 32-bit wide and 16-byte aligned.
+	 * Compensate for the resulting bit number spacing.
+	 */
+	return vec + 96 * (vec / 32);
+}
+
 static inline void apic_clear_vector(int vec, void *bitmap)
 {
-	clear_bit(APIC_VECTOR_TO_BIT_NUMBER(vec), bitmap + APIC_VECTOR_TO_REG_OFFSET(vec));
+	clear_bit(get_vec_bit(vec), bitmap);
 }
 
 static inline void apic_set_vector(int vec, void *bitmap)
 {
-	set_bit(APIC_VECTOR_TO_BIT_NUMBER(vec), bitmap + APIC_VECTOR_TO_REG_OFFSET(vec));
+	set_bit(get_vec_bit(vec), bitmap);
 }
 
 static inline int apic_test_vector(int vec, void *bitmap)
 {
-	return test_bit(APIC_VECTOR_TO_BIT_NUMBER(vec), bitmap + APIC_VECTOR_TO_REG_OFFSET(vec));
+	return test_bit(get_vec_bit(vec), bitmap);
 }
 
 /*
-- 
2.34.1

On Tue, 2025-06-10 at 23:24 +0530, Neeraj Upadhyay wrote:
> Use 'regs' as a contiguous linear bitmap in  apic_{set|
					      ^
					      double whitespace here

> clear|test}_vector() while doing bitwise operations.
> This makes the code simpler by eliminating the need to
> determine the offset of the 32-bit register and the vector
> bit location within that register prior to performing
> bitwise operations.
> 
> This change results in slight increase in generated code
> size for gcc-14.2.

Seems the text wrap here is different from other patches, i.e., the
'textwidth' seems much smaller.

> 
> - Without change
> 
> apic_set_vector:

Perhaps add a blank line here to make it easier to read (also consistent
with below).

> 89 f8             mov    %edi,%eax
> 83 e7 1f          and    $0x1f,%edi
> c1 e8 05          shr    $0x5,%eax
> c1 e0 04          shl    $0x4,%eax
> 48 01 c6          add    %rax,%rsi
> f0 48 0f ab 3e    lock bts %rdi,(%rsi)
> c3                ret
> 
> - With change
> 
> apic_set_vector:
> 
> 89 f8             mov    %edi,%eax
> c1 e8 05          shr    $0x5,%eax
> 8d 04 40          lea    (%rax,%rax,2),%eax
> c1 e0 05          shl    $0x5,%eax
> 01 f8             add    %edi,%eax
> 89 c0             mov    %eax,%eax
> f0 48 0f ab 3e    lock bts %rax,(%rsi)
> c3                ret
> 
> But, lapic.o text size (bytes) decreases with this change:
> 
> Obj        Old-size      New-size
> 
> lapic.o    28832         28768
> 
> Signed-off-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>

On Tue, Jun 24, 2025, Kai Huang wrote:
> On Tue, 2025-06-10 at 23:24 +0530, Neeraj Upadhyay wrote:
> > Use 'regs' as a contiguous linear bitmap in  apic_{set|
> 					      ^
> 					      double whitespace here
> 
> > clear|test}_vector() while doing bitwise operations.
> > This makes the code simpler by eliminating the need to
> > determine the offset of the 32-bit register and the vector
> > bit location within that register prior to performing
> > bitwise operations.
> > 
> > This change results in slight increase in generated code
> > size for gcc-14.2.
> 
> Seems the text wrap here is different from other patches, i.e., the
> 'textwidth' seems much smaller.

Yeah, wrap closer to ~75 chars.

On 6/24/2025 4:07 PM, Huang, Kai wrote:
> On Tue, 2025-06-10 at 23:24 +0530, Neeraj Upadhyay wrote:
>> Use 'regs' as a contiguous linear bitmap in  apic_{set|
> 					      ^
> 					      double whitespace here
> 

Will fix.

>> clear|test}_vector() while doing bitwise operations.
>> This makes the code simpler by eliminating the need to
>> determine the offset of the 32-bit register and the vector
>> bit location within that register prior to performing
>> bitwise operations.
>>
>> This change results in slight increase in generated code
>> size for gcc-14.2.
> 
> Seems the text wrap here is different from other patches, i.e., the
> 'textwidth' seems much smaller.
> 

Will fix it.

>>
>> - Without change
>>
>> apic_set_vector:
> 
> Perhaps add a blank line here to make it easier to read (also consistent
> with below).
> 

Ack


Thanks for the review!


- Neeraj