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Date:   Tue, 28 Jun 2022 23:37:24 -0700
From:   "H. Peter Anvin" <hpa@zytor.com>
To:     Greg KH <gregkh@linuxfoundation.org>,
        Ajay Kaher <akaher@vmware.com>
CC:     bhelgaas@google.com, tglx@linutronix.de, mingo@redhat.com,
        bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org,
        linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
        stable@vger.kernel.org, rostedt@goodmis.org, namit@vmware.com,
        srivatsab@vmware.com, srivatsa@csail.mit.edu, amakhalov@vmware.com,
        anishs@vmware.com, vsirnapalli@vmware.com, er.ajay.kaher@gmail.com
Subject: Re: [PATCH] MMIO should have more priority then IO
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On June 28, 2022 11:12:41 PM PDT, Greg KH <gregkh@linuxfoundation=2Eorg> wr=
ote:
>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)=2E They require twice as many PCI accesses and PIO
>> instructions are serializing=2E As a result, MMIO should be preferred
>> when possible over PIO=2E
>>=20
>> Bare metal test result
>> 1 million reads using raw_pci_read() took:
>> PIO: 0=2E433153 Sec=2E
>> MMIO: 0=2E268792 Sec=2E
>>=20
>> Virtual Machine test result
>> 1 hundred thousand reads using raw_pci_read() took:
>> PIO: 12=2E809 Sec=2E
>> MMIO: took 8=2E517 Sec=2E
>>=20
>> Signed-off-by: Ajay Kaher <akaher@vmware=2Ecom>
>> ---
>>  arch/x86/pci/common=2Ec          |  8 ++++----
>>  1 files changed, 4 insertions(+), 4 deletions(-)
>>=20
>> diff --git a/arch/x86/pci/common=2Ec b/arch/x86/pci/common=2Ec
>> index 3507f456f=2E=2E0b3383d9c 100644
>> --- a/arch/x86/pci/common=2Ec
>> +++ b/arch/x86/pci/common=2Ec
>> @@ -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 d=
evfn,
>>  						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 =3D=3D 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;
>>  }
>> =20
>>  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 =3D=3D 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;
>>  }
>> =20
>> --=20
>> 2=2E30=2E0
>>=20
>
><formletter>
>
>This is not the correct way to submit patches for inclusion in the
>stable kernel tree=2E  Please read:
>    https://www=2Ekernel=2Eorg/doc/html/latest/process/stable-kernel-rule=
s=2Ehtml
>for how to do this properly=2E
>
></formletter>

The statement in the header is also incorrect=2E