[PATCH 0/8] x86/irq: KVM: Optimize KVM's PIR harvesting

[PATCH 0/8] x86/irq: KVM: Optimize KVM's PIR harvesting

[Sean Christopherson]

Optimizing KVM's PIR harvesting using the same techniques as posted MSIs,
most notably to use 8-byte accesses on 64-bit kernels (/facepalm).

Fix a few warts along the way, and finish up by adding a helper to dedup
the PIR harvesting code between KVM and posted MSIs.

Sean Christopherson (8):
  x86/irq: Ensure initial PIR loads are performed exactly once
  x86/irq: Track if IRQ was found in PIR during initial loop (to load
    PIR vals)
  KVM: VMX: Ensure vIRR isn't reloaded at odd times when sync'ing PIR
  x86/irq: KVM: Track PIR bitmap as an "unsigned long" array
  KVM: VMX: Process PIR using 64-bit accesses on 64-bit kernels
  KVM: VMX: Isolate pure loads from atomic XCHG when processing PIR
  KVM: VMX: Use arch_xchg() when processing PIR to avoid instrumentation
  x86/irq: KVM: Add helper for harvesting PIR to deduplicate KVM and
    posted MSIs

 arch/x86/include/asm/posted_intr.h | 79 +++++++++++++++++++++++++++---
 arch/x86/kernel/irq.c              | 63 ++++--------------------
 arch/x86/kvm/lapic.c               | 20 ++++----
 arch/x86/kvm/lapic.h               |  4 +-
 arch/x86/kvm/vmx/posted_intr.h     |  2 +-
 5 files changed, 96 insertions(+), 72 deletions(-)


base-commit: c9ea48bb6ee6b28bbc956c1e8af98044618fed5e
-- 
2.49.0.rc1.451.g8f38331e32-goog

[PATCH 1/8] x86/irq: Ensure initial PIR loads are performed exactly once

[Sean Christopherson]

Ensure the PIR is read exactly once at the start of handle_pending_pir(),
to guarantee that checking for an outstanding posted interrupt in a given
chuck doesn't reload the chunk from the "real" PIR.  Functionally, a reload
is benign, but it would defeat the purpose of pre-loading into a copy.

Fixes: 1b03d82ba15e ("x86/irq: Install posted MSI notification handler")
Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kernel/irq.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 385e3a5fc304..9e5263887ff6 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -412,7 +412,7 @@ static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
 	bool handled = false;
 
 	for (i = 0; i < 4; i++)
-		pir_copy[i] = pir[i];
+		pir_copy[i] = READ_ONCE(pir[i]);
 
 	for (i = 0; i < 4; i++) {
 		if (!pir_copy[i])
-- 
2.49.0.rc1.451.g8f38331e32-goog

Re: [PATCH 1/8] x86/irq: Ensure initial PIR loads are performed exactly once

[Thomas Gleixner]

On Fri, Mar 14 2025 at 20:06, Sean Christopherson wrote:
> Ensure the PIR is read exactly once at the start of handle_pending_pir(),
> to guarantee that checking for an outstanding posted interrupt in a given
> chuck doesn't reload the chunk from the "real" PIR.  Functionally, a reload
> is benign, but it would defeat the purpose of pre-loading into a copy.
>
> Fixes: 1b03d82ba15e ("x86/irq: Install posted MSI notification handler")
> Cc: Jacob Pan <jacob.jun.pan@linux.intel.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

[PATCH 2/8] x86/irq: Track if IRQ was found in PIR during initial loop (to load PIR vals)

[Sean Christopherson]

Track whether or not at least one IRQ was found in PIR during the initial
loop to load PIR chunks from memory.  Doing so generates slightly better
code (arguably), especially for the case where there are no pending IRQs.

Note, while PIR can be modified between the initial load and the XCHG, it
can only _gain_ new IRQs, i.e. there is no danger of a false positive due
to the final version of pir_copy[] being empty.

Opportunistically rename the boolean in anticipation of moving the PIR
accesses to a common helper that can be shared by posted MSIs and KVM.

Old:
   <+74>:	test   %rdx,%rdx
   <+77>:	je     0xffffffff812bbeb0 <handle_pending_pir+144>
   <pir[0]>
   <+88>:	mov    $0x1,%dl>
   <+90>:	test   %rsi,%rsi
   <+93>:	je     0xffffffff812bbe8c <handle_pending_pir+108>
   <pir[1]>
   <+106>:	mov    $0x1,%dl
   <+108>:	test   %rcx,%rcx
   <+111>:	je     0xffffffff812bbe9e <handle_pending_pir+126>
   <pir[2]>
   <+124>:	mov    $0x1,%dl
   <+126>:	test   %rax,%rax
   <+129>:	je     0xffffffff812bbeb9 <handle_pending_pir+153>
   <pir[3]>
   <+142>:	jmp    0xffffffff812bbec1 <handle_pending_pir+161>
   <+144>:	xor    %edx,%edx
   <+146>:	test   %rsi,%rsi
   <+149>:	jne    0xffffffff812bbe7f <handle_pending_pir+95>
   <+151>:	jmp    0xffffffff812bbe8c <handle_pending_pir+108>
   <+153>:	test   %dl,%dl
   <+155>:	je     0xffffffff812bbf8e <handle_pending_pir+366>

New:
   <+74>:	mov    %rax,%r8
   <+77>:	or     %rcx,%r8
   <+80>:	or     %rdx,%r8
   <+83>:	or     %rsi,%r8
   <+86>:	setne  %bl
   <+89>:	je     0xffffffff812bbf88 <handle_pending_pir+360>
   <+95>:	test   %rsi,%rsi
   <+98>:	je     0xffffffff812bbe8d <handle_pending_pir+109>
   <pir[0]>
   <+109>:	test   %rdx,%rdx
   <+112>:	je     0xffffffff812bbe9d <handle_pending_pir+125>
   <pir[1]>
   <+125>:	test   %rcx,%rcx
   <+128>:	je     0xffffffff812bbead <handle_pending_pir+141>
   <pir[2]>
   <+141>:	test   %rax,%rax
   <+144>:	je     0xffffffff812bbebd <handle_pending_pir+157>
   <pir[3]>

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kernel/irq.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 9e5263887ff6..3f95b00ccd7f 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -409,25 +409,28 @@ static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
 {
 	int i, vec = FIRST_EXTERNAL_VECTOR;
 	unsigned long pir_copy[4];
-	bool handled = false;
+	bool found_irq = false;
 
-	for (i = 0; i < 4; i++)
+	for (i = 0; i < 4; i++) {
 		pir_copy[i] = READ_ONCE(pir[i]);
+		if (pir_copy[i])
+			found_irq = true;
+	}
+
+	if (!found_irq)
+		return false;
 
 	for (i = 0; i < 4; i++) {
 		if (!pir_copy[i])
 			continue;
 
 		pir_copy[i] = arch_xchg(&pir[i], 0);
-		handled = true;
 	}
 
-	if (handled) {
-		for_each_set_bit_from(vec, pir_copy, FIRST_SYSTEM_VECTOR)
-			call_irq_handler(vec, regs);
-	}
+	for_each_set_bit_from(vec, pir_copy, FIRST_SYSTEM_VECTOR)
+		call_irq_handler(vec, regs);
 
-	return handled;
+	return true;
 }
 
 /*
-- 
2.49.0.rc1.451.g8f38331e32-goog

Re: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during initial loop (to load PIR vals)

[Thomas Gleixner]

On Fri, Mar 14 2025 at 20:06, Sean Christopherson wrote:
> @@ -409,25 +409,28 @@ static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
>  {
>  	int i, vec = FIRST_EXTERNAL_VECTOR;
>  	unsigned long pir_copy[4];
> -	bool handled = false;
> +	bool found_irq = false;
>  
> -	for (i = 0; i < 4; i++)
> +	for (i = 0; i < 4; i++) {
>  		pir_copy[i] = READ_ONCE(pir[i]);
> +		if (pir_copy[i])
> +			found_irq = true;
> +	}

That's four extra conditional branches. You can avoid them completely. See
delta patch below.

Thanks,

        tglx
---        
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -407,17 +407,15 @@ void intel_posted_msi_init(void)
  */
 static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
 {
+	unsigned long pir_copy[4], pend = 0;
 	int i, vec = FIRST_EXTERNAL_VECTOR;
-	unsigned long pir_copy[4];
-	bool found_irq = false;
 
 	for (i = 0; i < 4; i++) {
 		pir_copy[i] = READ_ONCE(pir[i]);
-		if (pir_copy[i])
-			found_irq = true;
+		pend |= pir_copy[i];
 	}
 
-	if (!found_irq)
+	if (!pend)
 		return false;
 
 	for (i = 0; i < 4; i++) {

Re: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during initial loop (to load PIR vals)

[Sean Christopherson]

On Mon, Mar 17, 2025, Thomas Gleixner wrote:
> On Fri, Mar 14 2025 at 20:06, Sean Christopherson wrote:
> > @@ -409,25 +409,28 @@ static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
> >  {
> >  	int i, vec = FIRST_EXTERNAL_VECTOR;
> >  	unsigned long pir_copy[4];
> > -	bool handled = false;
> > +	bool found_irq = false;
> >  
> > -	for (i = 0; i < 4; i++)
> > +	for (i = 0; i < 4; i++) {
> >  		pir_copy[i] = READ_ONCE(pir[i]);
> > +		if (pir_copy[i])
> > +			found_irq = true;
> > +	}
> 
> That's four extra conditional branches. You can avoid them completely. See
> delta patch below.

Huh.  gcc elides the conditional branches when computing found_irq regardless of
the approach; the JEs in the changelog are from skipping the XCHG.

But clang-14 does not.  I'll slot this in.

Thanks!

Re: [PATCH 2/8] x86/irq: Track if IRQ was found in PIR during initial loop (to load PIR vals)

[Thomas Gleixner]

On Mon, Mar 17 2025 at 09:53, Sean Christopherson wrote:
> On Mon, Mar 17, 2025, Thomas Gleixner wrote:
>> > -	for (i = 0; i < 4; i++)
>> > +	for (i = 0; i < 4; i++) {
>> >  		pir_copy[i] = READ_ONCE(pir[i]);
>> > +		if (pir_copy[i])
>> > +			found_irq = true;
>> > +	}
>> 
>> That's four extra conditional branches. You can avoid them completely. See
>> delta patch below.
>
> Huh.  gcc elides the conditional branches when computing found_irq regardless of
> the approach; the JEs in the changelog are from skipping the XCHG.
>
> But clang-14 does not.  I'll slot this in.

Neither does GCC 12. That's why I noticed.

[PATCH 3/8] KVM: VMX: Ensure vIRR isn't reloaded at odd times when sync'ing PIR

[Sean Christopherson]

Read each vIRR exactly once when shuffling IRQs from the PIR to the vAPIC
to ensure getting the highest priority IRQ from the chunk doesn't reload
from the vIRR.  In practice, a reload is functionally benign as vcpu->mutex
is held and so IRQs can be consumed, i.e. new IRQs can appear, but existing
IRQs can't disappear.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/lapic.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 9dbc0f5d9865..cb4aeab914eb 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -667,7 +667,7 @@ bool __kvm_apic_update_irr(u32 *pir, void *regs, int *max_irr)
 	for (i = vec = 0; i <= 7; i++, vec += 32) {
 		u32 *p_irr = (u32 *)(regs + APIC_IRR + i * 0x10);
 
-		irr_val = *p_irr;
+		irr_val = READ_ONCE(*p_irr);
 		pir_val = READ_ONCE(pir[i]);
 
 		if (pir_val) {
-- 
2.49.0.rc1.451.g8f38331e32-goog

[PATCH 4/8] x86/irq: KVM: Track PIR bitmap as an "unsigned long" array

[Sean Christopherson]

Track the PIR bitmap in posted interrupt descriptor structures as an array
of unsigned longs instead of using unionized arrays for KVM (u32s) versus
IRQ management (u64s).  In practice, because the non-KVM usage is (sanely)
restricted to 64-bit kernels, all existing usage of the u64 variant is
already working with unsigned longs.

Using "unsigned long" for the array will allow reworking KVM's processing
of the bitmap to read/write in 64-bit chunks on 64-bit kernels, i.e. will
allow optimizing KVM by reducing the number of atomic accesses to PIR.

Opportunstically replace the open coded literals in the posted MSIs code
with the appropriate macro.  Deliberately don't use ARRAY_SIZE() in the
for-loops, even though it would be cleaner from a certain perspective, in
anticipation of decoupling the processing from the array declaration.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/posted_intr.h | 14 +++++++-------
 arch/x86/kernel/irq.c              | 12 ++++++------
 arch/x86/kvm/lapic.c               |  9 +++++----
 arch/x86/kvm/lapic.h               |  4 ++--
 arch/x86/kvm/vmx/posted_intr.h     |  2 +-
 5 files changed, 21 insertions(+), 20 deletions(-)

diff --git a/arch/x86/include/asm/posted_intr.h b/arch/x86/include/asm/posted_intr.h
index de788b400fba..c3e6e4221a5b 100644
--- a/arch/x86/include/asm/posted_intr.h
+++ b/arch/x86/include/asm/posted_intr.h
@@ -8,12 +8,12 @@
 
 #define PID_TABLE_ENTRY_VALID 1
 
+#define NR_PIR_VECTORS	256
+#define NR_PIR_WORDS	(NR_PIR_VECTORS / BITS_PER_LONG)
+
 /* Posted-Interrupt Descriptor */
 struct pi_desc {
-	union {
-		u32 pir[8];     /* Posted interrupt requested */
-		u64 pir64[4];
-	};
+	unsigned long pir[NR_PIR_WORDS];     /* Posted interrupt requested */
 	union {
 		struct {
 			u16	notifications; /* Suppress and outstanding bits */
@@ -43,12 +43,12 @@ static inline bool pi_test_and_clear_sn(struct pi_desc *pi_desc)
 
 static inline bool pi_test_and_set_pir(int vector, struct pi_desc *pi_desc)
 {
-	return test_and_set_bit(vector, (unsigned long *)pi_desc->pir);
+	return test_and_set_bit(vector, pi_desc->pir);
 }
 
 static inline bool pi_is_pir_empty(struct pi_desc *pi_desc)
 {
-	return bitmap_empty((unsigned long *)pi_desc->pir, NR_VECTORS);
+	return bitmap_empty(pi_desc->pir, NR_VECTORS);
 }
 
 static inline void pi_set_sn(struct pi_desc *pi_desc)
@@ -105,7 +105,7 @@ static inline bool pi_pending_this_cpu(unsigned int vector)
 	if (WARN_ON_ONCE(vector > NR_VECTORS || vector < FIRST_EXTERNAL_VECTOR))
 		return false;
 
-	return test_bit(vector, (unsigned long *)pid->pir);
+	return test_bit(vector, pid->pir);
 }
 
 extern void intel_posted_msi_init(void);
diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 3f95b00ccd7f..704c104ff7a4 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -405,13 +405,13 @@ void intel_posted_msi_init(void)
  * instead of:
  *		read, xchg, read, xchg, read, xchg, read, xchg
  */
-static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
+static __always_inline bool handle_pending_pir(unsigned long *pir, struct pt_regs *regs)
 {
 	int i, vec = FIRST_EXTERNAL_VECTOR;
-	unsigned long pir_copy[4];
+	unsigned long pir_copy[NR_PIR_WORDS];
 	bool found_irq = false;
 
-	for (i = 0; i < 4; i++) {
+	for (i = 0; i < NR_PIR_WORDS; i++) {
 		pir_copy[i] = READ_ONCE(pir[i]);
 		if (pir_copy[i])
 			found_irq = true;
@@ -420,7 +420,7 @@ static __always_inline bool handle_pending_pir(u64 *pir, struct pt_regs *regs)
 	if (!found_irq)
 		return false;
 
-	for (i = 0; i < 4; i++) {
+	for (i = 0; i < NR_PIR_WORDS; i++) {
 		if (!pir_copy[i])
 			continue;
 
@@ -460,7 +460,7 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_posted_msi_notification)
 	 * MAX_POSTED_MSI_COALESCING_LOOP - 1 loops are executed here.
 	 */
 	while (++i < MAX_POSTED_MSI_COALESCING_LOOP) {
-		if (!handle_pending_pir(pid->pir64, regs))
+		if (!handle_pending_pir(pid->pir, regs))
 			break;
 	}
 
@@ -475,7 +475,7 @@ DEFINE_IDTENTRY_SYSVEC(sysvec_posted_msi_notification)
 	 * process PIR bits one last time such that handling the new interrupts
 	 * are not delayed until the next IRQ.
 	 */
-	handle_pending_pir(pid->pir64, regs);
+	handle_pending_pir(pid->pir, regs);
 
 	apic_eoi();
 	irq_exit();
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index cb4aeab914eb..893e7d06e0e6 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -655,8 +655,9 @@ static u8 count_vectors(void *bitmap)
 	return count;
 }
 
-bool __kvm_apic_update_irr(u32 *pir, void *regs, int *max_irr)
+bool __kvm_apic_update_irr(unsigned long *pir, void *regs, int *max_irr)
 {
+	u32 *__pir = (void *)pir;
 	u32 i, vec;
 	u32 pir_val, irr_val, prev_irr_val;
 	int max_updated_irr;
@@ -668,10 +669,10 @@ bool __kvm_apic_update_irr(u32 *pir, void *regs, int *max_irr)
 		u32 *p_irr = (u32 *)(regs + APIC_IRR + i * 0x10);
 
 		irr_val = READ_ONCE(*p_irr);
-		pir_val = READ_ONCE(pir[i]);
+		pir_val = READ_ONCE(__pir[i]);
 
 		if (pir_val) {
-			pir_val = xchg(&pir[i], 0);
+			pir_val = xchg(&__pir[i], 0);
 
 			prev_irr_val = irr_val;
 			do {
@@ -691,7 +692,7 @@ bool __kvm_apic_update_irr(u32 *pir, void *regs, int *max_irr)
 }
 EXPORT_SYMBOL_GPL(__kvm_apic_update_irr);
 
-bool kvm_apic_update_irr(struct kvm_vcpu *vcpu, u32 *pir, int *max_irr)
+bool kvm_apic_update_irr(struct kvm_vcpu *vcpu, unsigned long *pir, int *max_irr)
 {
 	struct kvm_lapic *apic = vcpu->arch.apic;
 	bool irr_updated = __kvm_apic_update_irr(pir, apic->regs, max_irr);
diff --git a/arch/x86/kvm/lapic.h b/arch/x86/kvm/lapic.h
index 1a8553ebdb42..0d41780852e4 100644
--- a/arch/x86/kvm/lapic.h
+++ b/arch/x86/kvm/lapic.h
@@ -101,8 +101,8 @@ bool kvm_apic_match_dest(struct kvm_vcpu *vcpu, struct kvm_lapic *source,
 			   int shorthand, unsigned int dest, int dest_mode);
 int kvm_apic_compare_prio(struct kvm_vcpu *vcpu1, struct kvm_vcpu *vcpu2);
 void kvm_apic_clear_irr(struct kvm_vcpu *vcpu, int vec);
-bool __kvm_apic_update_irr(u32 *pir, void *regs, int *max_irr);
-bool kvm_apic_update_irr(struct kvm_vcpu *vcpu, u32 *pir, int *max_irr);
+bool __kvm_apic_update_irr(unsigned long *pir, void *regs, int *max_irr);
+bool kvm_apic_update_irr(struct kvm_vcpu *vcpu, unsigned long *pir, int *max_irr);
 void kvm_apic_update_ppr(struct kvm_vcpu *vcpu);
 int kvm_apic_set_irq(struct kvm_vcpu *vcpu, struct kvm_lapic_irq *irq,
 		     struct dest_map *dest_map);
diff --git a/arch/x86/kvm/vmx/posted_intr.h b/arch/x86/kvm/vmx/posted_intr.h
index ad9116a99bcc..4ff9d720dec0 100644
--- a/arch/x86/kvm/vmx/posted_intr.h
+++ b/arch/x86/kvm/vmx/posted_intr.h
@@ -18,7 +18,7 @@ static inline int pi_find_highest_vector(struct pi_desc *pi_desc)
 {
 	int vec;
 
-	vec = find_last_bit((unsigned long *)pi_desc->pir, 256);
+	vec = find_last_bit(pi_desc->pir, 256);
 	return vec < 256 ? vec : -1;
 }
 
-- 
2.49.0.rc1.451.g8f38331e32-goog

[PATCH 5/8] KVM: VMX: Process PIR using 64-bit accesses on 64-bit kernels

[Sean Christopherson]

Process the PIR at the natural kernel width, i.e. in 64-bit chunks on
64-bit kernels, so that the worst case of having a posted IRQ in each
chunk of the vIRR only requires 4 loads and xchgs from/to the PIR, not 8.

Deliberately use a "continue" to skip empty entries so that the code is a
carbon copy of handle_pending_pir(), in anticipation of deduplicating KVM
and posted MSI logic.

Suggested-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/lapic.c | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 893e7d06e0e6..e4f182ee9340 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -657,26 +657,32 @@ static u8 count_vectors(void *bitmap)
 
 bool __kvm_apic_update_irr(unsigned long *pir, void *regs, int *max_irr)
 {
-	u32 *__pir = (void *)pir;
+	unsigned long pir_vals[NR_PIR_WORDS];
+	u32 *__pir = (void *)pir_vals;
 	u32 i, vec;
-	u32 pir_val, irr_val, prev_irr_val;
+	u32 irr_val, prev_irr_val;
 	int max_updated_irr;
 
 	max_updated_irr = -1;
 	*max_irr = -1;
 
+	for (i = 0; i < NR_PIR_WORDS; i++) {
+		pir_vals[i] = READ_ONCE(pir[i]);
+		if (!pir_vals[i])
+			continue;
+
+		pir_vals[i] = xchg(&pir[i], 0);
+	}
+
 	for (i = vec = 0; i <= 7; i++, vec += 32) {
 		u32 *p_irr = (u32 *)(regs + APIC_IRR + i * 0x10);
 
 		irr_val = READ_ONCE(*p_irr);
-		pir_val = READ_ONCE(__pir[i]);
-
-		if (pir_val) {
-			pir_val = xchg(&__pir[i], 0);
 
+		if (__pir[i]) {
 			prev_irr_val = irr_val;
 			do {
-				irr_val = prev_irr_val | pir_val;
+				irr_val = prev_irr_val | __pir[i];
 			} while (prev_irr_val != irr_val &&
 				 !try_cmpxchg(p_irr, &prev_irr_val, irr_val));
 
-- 
2.49.0.rc1.451.g8f38331e32-goog

[PATCH 6/8] KVM: VMX: Isolate pure loads from atomic XCHG when processing PIR

[Sean Christopherson]

Rework KVM's processing of the PIR to use the same algorithm as posted
MSIs, i.e. to do READ(x4) => XCHG(x4) instead of (READ+XCHG)(x4).  Given
KVM's long-standing, sub-optimal use of 32-bit accesses to the PIR, it's
safe to say far more thought and investigation was put into handling the
PIR for posted MSIs, i.e. there's no reason to assume KVM's existing
logic is meaningful, let alone superior.

Matching the processing done by posted MSIs will also allow deduplicating
the code between KVM and posted MSIs.

See the comment for handle_pending_pir() added by commit 1b03d82ba15e
("x86/irq: Install posted MSI notification handler") for details on
why isolating loads from XCHG is desirable.

Suggested-by: Jim Mattson <jmattson@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/lapic.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index e4f182ee9340..d7e36faffc72 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -659,6 +659,7 @@ bool __kvm_apic_update_irr(unsigned long *pir, void *regs, int *max_irr)
 {
 	unsigned long pir_vals[NR_PIR_WORDS];
 	u32 *__pir = (void *)pir_vals;
+	bool found_irq = false;
 	u32 i, vec;
 	u32 irr_val, prev_irr_val;
 	int max_updated_irr;
@@ -668,6 +669,14 @@ bool __kvm_apic_update_irr(unsigned long *pir, void *regs, int *max_irr)
 
 	for (i = 0; i < NR_PIR_WORDS; i++) {
 		pir_vals[i] = READ_ONCE(pir[i]);
+		if (pir_vals[i])
+			found_irq = true;
+	}
+
+	if (!found_irq)
+		return false;
+
+	for (i = 0; i < NR_PIR_WORDS; i++) {
 		if (!pir_vals[i])
 			continue;
 
-- 
2.49.0.rc1.451.g8f38331e32-goog

[PATCH 7/8] KVM: VMX: Use arch_xchg() when processing PIR to avoid instrumentation

[Sean Christopherson]

Use arch_xchg() when moving IRQs from the PIR to the vIRR, purely to avoid
instrumentation so that KVM is compatible with the needs of posted MSI.
This will allow extracting the core PIR logic to common code and sharing
it between KVM and posted MSI handling.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/lapic.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index d7e36faffc72..b65e0f7223fe 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -680,7 +680,7 @@ bool __kvm_apic_update_irr(unsigned long *pir, void *regs, int *max_irr)
 		if (!pir_vals[i])
 			continue;
 
-		pir_vals[i] = xchg(&pir[i], 0);
+		pir_vals[i] = arch_xchg(&pir[i], 0);
 	}
 
 	for (i = vec = 0; i <= 7; i++, vec += 32) {
-- 
2.49.0.rc1.451.g8f38331e32-goog

[PATCH 8/8] x86/irq: KVM: Add helper for harvesting PIR to deduplicate KVM and posted MSIs

[Sean Christopherson]

Now that posted MSI and KVM harvesting of PIR is identical, extract the
code (and posted MSI's wonderful comment) to a common helper.

No functional change intended.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/include/asm/posted_intr.h | 65 ++++++++++++++++++++++++++++++
 arch/x86/kernel/irq.c              | 50 +----------------------
 arch/x86/kvm/lapic.c               | 16 +-------
 3 files changed, 68 insertions(+), 63 deletions(-)

diff --git a/arch/x86/include/asm/posted_intr.h b/arch/x86/include/asm/posted_intr.h
index c3e6e4221a5b..6b1ddebbf06a 100644
--- a/arch/x86/include/asm/posted_intr.h
+++ b/arch/x86/include/asm/posted_intr.h
@@ -1,8 +1,13 @@
 /* SPDX-License-Identifier: GPL-2.0 */
 #ifndef _X86_POSTED_INTR_H
 #define _X86_POSTED_INTR_H
+
+#include <asm/cmpxchg.h>
+#include <asm/rwonce.h>
 #include <asm/irq_vectors.h>
 
+#include <linux/bitmap.h>
+
 #define POSTED_INTR_ON  0
 #define POSTED_INTR_SN  1
 
@@ -26,6 +31,66 @@ struct pi_desc {
 	u32 rsvd[6];
 } __aligned(64);
 
+/*
+ * De-multiplexing posted interrupts is on the performance path, the code
+ * below is written to optimize the cache performance based on the following
+ * considerations:
+ * 1.Posted interrupt descriptor (PID) fits in a cache line that is frequently
+ *   accessed by both CPU and IOMMU.
+ * 2.During software processing of posted interrupts, the CPU needs to do
+ *   natural width read and xchg for checking and clearing posted interrupt
+ *   request (PIR), a 256 bit field within the PID.
+ * 3.On the other side, the IOMMU does atomic swaps of the entire PID cache
+ *   line when posting interrupts and setting control bits.
+ * 4.The CPU can access the cache line a magnitude faster than the IOMMU.
+ * 5.Each time the IOMMU does interrupt posting to the PIR will evict the PID
+ *   cache line. The cache line states after each operation are as follows,
+ *   assuming a 64-bit kernel:
+ *   CPU		IOMMU			PID Cache line state
+ *   ---------------------------------------------------------------
+ *...read64					exclusive
+ *...lock xchg64				modified
+ *...			post/atomic swap	invalid
+ *...-------------------------------------------------------------
+ *
+ * To reduce L1 data cache miss, it is important to avoid contention with
+ * IOMMU's interrupt posting/atomic swap. Therefore, a copy of PIR is used
+ * when processing posted interrupts in software, e.g. to dispatch interrupt
+ * handlers for posted MSIs, or to move interrupts from the PIR to the vIRR
+ * in KVM.
+ *
+ * In addition, the code is trying to keep the cache line state consistent
+ * as much as possible. e.g. when making a copy and clearing the PIR
+ * (assuming non-zero PIR bits are present in the entire PIR), it does:
+ *		read, read, read, read, xchg, xchg, xchg, xchg
+ * instead of:
+ *		read, xchg, read, xchg, read, xchg, read, xchg
+ */
+static __always_inline bool pi_harvest_pir(unsigned long *pir,
+					   unsigned long *pir_vals)
+{
+	bool found_irq = false;
+	int i;
+
+	for (i = 0; i < NR_PIR_WORDS; i++) {
+		pir_vals[i] = READ_ONCE(pir[i]);
+		if (pir_vals[i])
+			found_irq = true;
+	}
+
+	if (!found_irq)
+		return false;
+
+	for (i = 0; i < NR_PIR_WORDS; i++) {
+		if (!pir_vals[i])
+			continue;
+
+		pir_vals[i] = arch_xchg(&pir[i], 0);
+	}
+
+	return true;
+}
+
 static inline bool pi_test_and_set_on(struct pi_desc *pi_desc)
 {
 	return test_and_set_bit(POSTED_INTR_ON, (unsigned long *)&pi_desc->control);
diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
index 704c104ff7a4..b98a5abdeaec 100644
--- a/arch/x86/kernel/irq.c
+++ b/arch/x86/kernel/irq.c
@@ -373,60 +373,14 @@ void intel_posted_msi_init(void)
 	this_cpu_write(posted_msi_pi_desc.ndst, destination);
 }
 
-/*
- * De-multiplexing posted interrupts is on the performance path, the code
- * below is written to optimize the cache performance based on the following
- * considerations:
- * 1.Posted interrupt descriptor (PID) fits in a cache line that is frequently
- *   accessed by both CPU and IOMMU.
- * 2.During posted MSI processing, the CPU needs to do 64-bit read and xchg
- *   for checking and clearing posted interrupt request (PIR), a 256 bit field
- *   within the PID.
- * 3.On the other side, the IOMMU does atomic swaps of the entire PID cache
- *   line when posting interrupts and setting control bits.
- * 4.The CPU can access the cache line a magnitude faster than the IOMMU.
- * 5.Each time the IOMMU does interrupt posting to the PIR will evict the PID
- *   cache line. The cache line states after each operation are as follows:
- *   CPU		IOMMU			PID Cache line state
- *   ---------------------------------------------------------------
- *...read64					exclusive
- *...lock xchg64				modified
- *...			post/atomic swap	invalid
- *...-------------------------------------------------------------
- *
- * To reduce L1 data cache miss, it is important to avoid contention with
- * IOMMU's interrupt posting/atomic swap. Therefore, a copy of PIR is used
- * to dispatch interrupt handlers.
- *
- * In addition, the code is trying to keep the cache line state consistent
- * as much as possible. e.g. when making a copy and clearing the PIR
- * (assuming non-zero PIR bits are present in the entire PIR), it does:
- *		read, read, read, read, xchg, xchg, xchg, xchg
- * instead of:
- *		read, xchg, read, xchg, read, xchg, read, xchg
- */
 static __always_inline bool handle_pending_pir(unsigned long *pir, struct pt_regs *regs)
 {
-	int i, vec = FIRST_EXTERNAL_VECTOR;
+	int vec = FIRST_EXTERNAL_VECTOR;
 	unsigned long pir_copy[NR_PIR_WORDS];
-	bool found_irq = false;
 
-	for (i = 0; i < NR_PIR_WORDS; i++) {
-		pir_copy[i] = READ_ONCE(pir[i]);
-		if (pir_copy[i])
-			found_irq = true;
-	}
-
-	if (!found_irq)
+	if (!pi_harvest_pir(pir, pir_copy))
 		return false;
 
-	for (i = 0; i < NR_PIR_WORDS; i++) {
-		if (!pir_copy[i])
-			continue;
-
-		pir_copy[i] = arch_xchg(&pir[i], 0);
-	}
-
 	for_each_set_bit_from(vec, pir_copy, FIRST_SYSTEM_VECTOR)
 		call_irq_handler(vec, regs);
 
diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index b65e0f7223fe..1c611b84b8ab 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -659,7 +659,6 @@ bool __kvm_apic_update_irr(unsigned long *pir, void *regs, int *max_irr)
 {
 	unsigned long pir_vals[NR_PIR_WORDS];
 	u32 *__pir = (void *)pir_vals;
-	bool found_irq = false;
 	u32 i, vec;
 	u32 irr_val, prev_irr_val;
 	int max_updated_irr;
@@ -667,22 +666,9 @@ bool __kvm_apic_update_irr(unsigned long *pir, void *regs, int *max_irr)
 	max_updated_irr = -1;
 	*max_irr = -1;
 
-	for (i = 0; i < NR_PIR_WORDS; i++) {
-		pir_vals[i] = READ_ONCE(pir[i]);
-		if (pir_vals[i])
-			found_irq = true;
-	}
-
-	if (!found_irq)
+	if (!pi_harvest_pir(pir, pir_vals))
 		return false;
 
-	for (i = 0; i < NR_PIR_WORDS; i++) {
-		if (!pir_vals[i])
-			continue;
-
-		pir_vals[i] = arch_xchg(&pir[i], 0);
-	}
-
 	for (i = vec = 0; i <= 7; i++, vec += 32) {
 		u32 *p_irr = (u32 *)(regs + APIC_IRR + i * 0x10);
 
-- 
2.49.0.rc1.451.g8f38331e32-goog