linux-kernel - [PATCH] MMIO should have more priority then IO

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Date:   Tue, 28 Jun 2022 21:59:21 +0530
From:   Ajay Kaher <akaher@...are.com>
To:     bhelgaas@...gle.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com
Cc:     x86@...nel.org, hpa@...or.com, linux-pci@...r.kernel.org,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        rostedt@...dmis.org, namit@...are.com, srivatsab@...are.com,
        srivatsa@...il.mit.edu, amakhalov@...are.com, anishs@...are.com,
        vsirnapalli@...are.com, akaher@...are.com, er.ajay.kaher@...il.com
Subject: [PATCH] MMIO should have more priority then IO 

Port IO instructions (PIO) are less efficient than MMIO (memory
mapped I/O). They require twice as many PCI accesses and PIO
instructions are serializing. As a result, MMIO should be preferred
when possible over PIO.

Bare metal test result
1 million reads using raw_pci_read() took:
PIO: 0.433153 Sec.
MMIO: 0.268792 Sec.

Virtual Machine test result
1 hundred thousand reads using raw_pci_read() took:
PIO: 12.809 Sec.
MMIO: took 8.517 Sec.

Signed-off-by: Ajay Kaher <akaher@...are.com>
---
 arch/x86/pci/common.c          |  8 ++++----
 1 files changed, 4 insertions(+), 4 deletions(-)

diff --git a/arch/x86/pci/common.c b/arch/x86/pci/common.c
index 3507f456f..0b3383d9c 100644
--- a/arch/x86/pci/common.c
+++ b/arch/x86/pci/common.c
@@ -40,20 +40,20 @@ const struct pci_raw_ops *__read_mostly raw_pci_ext_ops;
 int raw_pci_read(unsigned int domain, unsigned int bus, unsigned int devfn,
 						int reg, int len, u32 *val)
 {
+	if (raw_pci_ext_ops)
+		return raw_pci_ext_ops->read(domain, bus, devfn, reg, len, val);
 	if (domain == 0 && reg < 256 && raw_pci_ops)
 		return raw_pci_ops->read(domain, bus, devfn, reg, len, val);
-	if (raw_pci_ext_ops)
-		return raw_pci_ext_ops->read(domain, bus, devfn, reg, len, val);
 	return -EINVAL;
 }
 
 int raw_pci_write(unsigned int domain, unsigned int bus, unsigned int devfn,
 						int reg, int len, u32 val)
 {
+	if (raw_pci_ext_ops)
+		return raw_pci_ext_ops->write(domain, bus, devfn, reg, len, val);
 	if (domain == 0 && reg < 256 && raw_pci_ops)
 		return raw_pci_ops->write(domain, bus, devfn, reg, len, val);
-	if (raw_pci_ext_ops)
-		return raw_pci_ext_ops->write(domain, bus, devfn, reg, len, val);
 	return -EINVAL;
 }
 
-- 
2.30.0