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

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Date:   Tue, 28 Jun 2022 23:37:24 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Greg KH <gregkh@...uxfoundation.org>,
        Ajay Kaher <akaher@...are.com>
CC:     bhelgaas@...gle.com, tglx@...utronix.de, mingo@...hat.com,
        bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org,
        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, er.ajay.kaher@...il.com
Subject: Re: [PATCH] MMIO should have more priority then IO

On June 28, 2022 11:12:41 PM PDT, Greg KH <gregkh@...uxfoundation.org> wrote:
>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@...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
>> 
>
><formletter>
>
>This is not the correct way to submit patches for inclusion in the
>stable kernel tree.  Please read:
>    https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html
>for how to do this properly.
>
></formletter>

The statement in the header is also incorrect.