[+cc Matthew] On Tue, Jun 28, 2022 at 09:59:21PM +0530, Ajay Kaher wrote: > 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@xxxxxxxxxx> > --- > 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; This organization of raw_pci_read() dates to b6ce068a1285 ("Change pci_raw_ops to pci_raw_read/write"), by Matthew. Cc'd him for comment, since I think he considered the ordering at the time. > } > > 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 >