linux-kernel - Re: [PATCH 1/1] PCI: Fix Extend ACS configurability

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Message-ID: <2676cf6e-d9eb-4a34-be5e-29824458f92f@nvidia.com>
Date: Mon, 6 Jan 2025 12:34:00 -0800
From: Tushar Dave <tdave@...dia.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: corbet@....net, bhelgaas@...gle.com, paulmck@...nel.org,
 akpm@...ux-foundation.org, thuth@...hat.com, rostedt@...dmis.org,
 xiongwei.song@...driver.com, vidyas@...dia.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org, vsethi@...dia.com,
 sdonthineni@...dia.com
Subject: Re: [PATCH 1/1] PCI: Fix Extend ACS configurability



On 1/2/25 10:40, Jason Gunthorpe wrote:
> On Fri, Dec 13, 2024 at 12:29:42PM -0800, Tushar Dave wrote:
> 
>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>> index dc663c0ca670..fc1c37910d1c 100644
>> --- a/Documentation/admin-guide/kernel-parameters.txt
>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>> @@ -4654,11 +4654,10 @@
>>   				Format:
>>   				<ACS flags>@<pci_dev>[; ...]
>>   				Specify one or more PCI devices (in the format
>> -				specified above) optionally prepended with flags
>> -				and separated by semicolons. The respective
>> -				capabilities will be enabled, disabled or
>> -				unchanged based on what is specified in
>> -				flags.
>> +				specified above) prepended with flags and separated
>> +				by semicolons. The respective capabilities will be
>> +				enabled, disabled or unchanged based on what is
>> +				specified in flags.
>>   
>>   				ACS Flags is defined as follows:
>>   				  bit-0 : ACS Source Validation
>> @@ -4673,7 +4672,7 @@
>>   				  '1' – force enabled
>>   				  'x' – unchanged
>>   				For example,
>> -				  pci=config_acs=10x
>> +				  pci=config_acs=10x@pci:0:0
>>   				would configure all devices that support
>>   				ACS to enable P2P Request Redirect, disable
>>   				Translation Blocking, and leave Source
> 
> Is this an unrelated change? The format of the command line shouldn't
> be changed to fix the described bug, why is the documentation changed?

The documentation as it is (i.e. without my patch), is not correct.

IOW, config_acs parameter does require flags and it is not optional. Without 
flags it results into "ACS Flags missing". Therefore I remove word "optionally" 
from the documentation text.

Secondly, the syntax in the example 'pci=config_acs=10x' is incorrect. The 
correct syntax should be 'pci=config_acs=10x@pci:0:0' that would configure all 
devices that support ACS to enable P2P Request Redirect, disable Translation 
Blocking, and leave Source Validation unchanged from whatever power-up or 
firmware set it to.

> 
>>   static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
>> -			     const char *p, u16 mask, u16 flags)
>> +			     const char *p, const u16 acs_mask, const u16 acs_flags)
>>   {
>> +	u16 flags = acs_flags;
>> +	u16 mask = acs_mask;
>>   	char *delimit;
>>   	int ret = 0;
>>   
>> @@ -964,7 +965,7 @@ static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
>>   		return;
>>   
>>   	while (*p) {
>> -		if (!mask) {
>> +		if (!acs_mask) {
>>   			/* Check for ACS flags */
>>   			delimit = strstr(p, "@");
>>   			if (delimit) {
>> @@ -972,6 +973,8 @@ static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
>>   				u32 shift = 0;
>>   
>>   				end = delimit - p - 1;
>> +				mask = 0;
>> +				flags = 0;
>>   
>>   				while (end > -1) {
>>   					if (*(p + end) == '0') {
> 
> This function the entire fix, right? Because the routine was
> clobbering acs_mask as it processed the earlier devices?

yes, that is correct.

> 
>> @@ -1028,10 +1031,10 @@ static void __pci_config_acs(struct pci_dev *dev, struct pci_acs *caps,
>>   
>>   	pci_dbg(dev, "ACS mask  = %#06x\n", mask);
>>   	pci_dbg(dev, "ACS flags = %#06x\n", flags);
>> +	pci_dbg(dev, "ACS control = %#06x\n", caps->ctrl);
>>   
>> -	/* If mask is 0 then we copy the bit from the firmware setting. */
>> -	caps->ctrl = (caps->ctrl & ~mask) | (caps->fw_ctrl & mask);
>> -	caps->ctrl |= flags;
>> +	caps->ctrl &= ~mask;
>> +	caps->ctrl |= (flags & mask);
> 
> And why delete fw_ctrl? Doesn't that break the unchanged
> functionality?

No, it does not break the unchanged functionality. I removed it because it is 
not needed after my fix.

If it helps, using 'config_acs' the code only allows to configures the lower 7 
bits of ACS ctrl for the specified PCI device(s).
The bits other than the lower 7 bits of ACS ctrl remain unchanged.
The bits specified with 'x' or 'X' that are within the 7 lower bits remain 
unchanged. Trying to configure bits other than lower 7 bits generates an error 
message "Invalid ACS flags specified"

-Tushar
> 
> Jason