Subject: Re: [RFC PATCH v3 1/5] PCI: Add support for enforcing all MMIO BARs to be page aligned

From: Yongji Xie <[email-masked]>
Date: Mon, 1 Feb 2016 16:50:49 +0800

On 2016/1/30 3:01, Alex Williamson wrote:
> On Fri, 2016-01-29 at 18:37 +0800, Yongji Xie wrote:
>> On 2016/1/29 6:46, Alex Williamson wrote:
>>> On Fri, 2016-01-15 at 15:06 +0800, Yongji Xie wrote:
>>>> When vfio passthrough a PCI device of which MMIO BARs
>>>> are smaller than PAGE_SIZE, guest will not handle the
>>>> mmio accesses to the BARs which leads to mmio emulations
>>>> in host.
>>>>    
>>>> This is because vfio will not allow to passthrough one
>>>> BAR's mmio page which may be shared with other BARs.
>>>>    
>>>> To solve this performance issue, this patch adds a kernel
>>>> parameter "pci=resource_page_aligned=on" to enforce
>>>> the alignment of all MMIO BARs to be at least PAGE_SIZE,
>>>> so that one BAR's mmio page would not be shared with other
>>>> BARs. We can also disable it through kernel parameter
>>>> "pci=resource_page_aligned=off".
>>>>    
>>>> For the default value of the parameter, we think it should be
>>>> arch-independent, so we add a macro
>>>> HAVE_PCI_DEFAULT_RESOURCES_PAGE_ALIGNED to change it. And we
>>>> define this macro to enable this parameter by default on PPC64
>>>> platform which can easily hit this performance issue because
>>>> its PAGE_SIZE is 64KB.
>>>>    
>>>> Note that the kernel parameter won't works if kernel doesn't do
>>>> resources reallocation.
>>> And where do you account for this so that we know whether it's really in
>>> effect?
>>   
>> We can check the flag PCI_PROBE_ONLY to know whether kernel do
>> resources reallocation. Then we know if the kernel parameter is really
>> in effect.
>>   
>> enum {
>>       /* Force re-assigning all resources (ignore firmware
>>        * setup completely)
>>        */
>>       PCI_REASSIGN_ALL_RSRC    = 0x00000001,
>>   
>>       /* Re-assign all bus numbers */
>>       PCI_REASSIGN_ALL_BUS    = 0x00000002,
>>   
>>       /* Do not try to assign, just use existing setup */
>> --->    PCI_PROBE_ONLY        = 0x00000004,
>>   
>> And I will add this to commit log.
> We need more than a commit log entry for this, what's the purpose of the
> pci_resources_share_page() function if we don't know if this is in
> effect?

It seems the parameter will be always in effect if we reuse the 
re-aligning code
of parameter "resource_alignment=" in pci_reassigndev_resource_alignment().

>>>> Signed-off-by: Yongji Xie <[email-masked]>
>>>> ---
>>>>    Documentation/kernel-parameters.txt |    5 +++++
>>>>    arch/powerpc/include/asm/pci.h      |   11 +++++++++++
>>>>    drivers/pci/pci.c                   |   35 +++++++++++++++++++++++++++++++++++
>>>>    drivers/pci/pci.h                   |    8 +++++++-
>>>>    include/linux/pci.h                 |    4 ++++
>>>>    5 files changed, 62 insertions(+), 1 deletion(-)
>>>>    
>>>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>>>> index 742f69d..3f2a7c9 100644
>>>> --- a/Documentation/kernel-parameters.txt
>>>> +++ b/Documentation/kernel-parameters.txt
>>>> @@ -2857,6 +2857,11 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
>>>>    				PAGE_SIZE is used as alignment.
>>>>    				PCI-PCI bridge can be specified, if resource
>>>>    				windows need to be expanded.
>>>> +		resource_page_aligned=	Enable/disable enforcing the alignment
>>>> +				of all PCI devices' memory resources to be
>>>> +				at least PAGE_SIZE if resources reallocation
>>>> +				is done by kernel.
>>>> +				Format: { "on" | "off" }
>>>>    		ecrc=		Enable/disable PCIe ECRC (transaction layer
>>>>    				end-to-end CRC checking).
>>>>    				bios: Use BIOS/firmware settings. This is the
>>>> diff --git a/arch/powerpc/include/asm/pci.h b/arch/powerpc/include/asm/pci.h
>>>> index 3453bd8..2d2b3ef 100644
>>>> --- a/arch/powerpc/include/asm/pci.h
>>>> +++ b/arch/powerpc/include/asm/pci.h
>>>> @@ -136,6 +136,17 @@ extern pgprot_t	pci_phys_mem_access_prot(struct file *file,
>>>>    					 unsigned long pfn,
>>>>    					 unsigned long size,
>>>>    					 pgprot_t prot);
>>>> +#ifdef CONFIG_PPC64
>>>> +
>>>> +/* For PPC64, We enforce all PCI MMIO BARs to be page aligned
>>>> + * by default. This would be helpful to improve performance
>>>> + * when we passthrough a PCI device of which BARs are smaller
>>>> + * than PAGE_SIZE(64KB). And we can use kernel parameter
>>>> + * "pci=resource_page_aligned=off" to disable it.
>>>> + */
>>>> +#define HAVE_PCI_DEFAULT_RESOURCES_PAGE_ALIGNED	1
>>>> +
>>>> +#endif
>>>>    
>>>>    #define HAVE_ARCH_PCI_RESOURCE_TO_USER
>>>>    extern void pci_resource_to_user(const struct pci_dev *dev, int bar,
>>>> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
>>>> index 314db8c..7b21238 100644
>>>> --- a/drivers/pci/pci.c
>>>> +++ b/drivers/pci/pci.c
>>>> @@ -99,6 +99,9 @@ u8 pci_cache_line_size;
>>>>     */
>>>>    unsigned int pcibios_max_latency = 255;
>>>>    
>>>> +bool pci_resources_page_aligned =
>>>> +	IS_ENABLED(HAVE_PCI_DEFAULT_RESOURCES_PAGE_ALIGNED);
>>> I don't think this is proper use of IS_ENABLED, which seems to be
>>> targeted at CONFIG_ type options.  You could define this as that in an
>>> arch Kconfig.
>>   
>> Is it better that we define this as a pci Kconfig and select it in arch
>> Kconfig?
> If you want to use IS_ENABLE here, I would think so.

Actually, I'm not sure if it's necessary to add a Kconfig option for it.

I prefer to do it like previous version:

#ifdef HAVE_PCI_DEFAULT_RESOURCES_PAGE_ALIGNED
bool pci_resources_page_aligned = true;
#else
bool pci_resources_page_aligned;
#endif

>>>> +
>>>>    /* If set, the PCIe ARI capability will not be used. */
>>>>    static bool pcie_ari_disabled;
>>>>    
>>>> @@ -4746,6 +4749,35 @@ static ssize_t pci_resource_alignment_store(struct bus_type *bus,
>>>>    BUS_ATTR(resource_alignment, 0644, pci_resource_alignment_show,
>>>>    					pci_resource_alignment_store);
>>>>    
>>>> +static void pci_resources_get_page_aligned(char *str)
>>>> +{
>>>> +	if (!strncmp(str, "off", 3))
>>>> +		pci_resources_page_aligned = false;
>>>> +	else if (!strncmp(str, "on", 2))
>>>> +		pci_resources_page_aligned = true;
>>>> +}
>>> "get"?
>>   
>> How about pci_resources_parse_page_aligned_param()?
> Better.
>
>>>> +
>>>> +/*
>>>> + * This function checks whether PCI BARs' mmio page will be shared
>>>> + * with other BARs.
>>>> + */
>>>> +bool pci_resources_share_page(struct pci_dev *dev, int resno)
>>>> +{
>>>> +	struct resource *res = dev->resource + resno;
>>>> +
>>>> +	if (resource_size(res) >= PAGE_SIZE)
>>>> +		return false;
>>>> +	if (pci_resources_page_aligned && !(res->start & ~PAGE_MASK) &&
>>>> +	    res->flags & IORESOURCE_MEM) {
>>>> +		if (res->sibling)
>>>> +			return (res->sibling->start & ~PAGE_MASK);
>>>> +		else
>>>> +			return false;
>>>> +	}
>>>> +	return true;
>>>> +}
>>>> +EXPORT_SYMBOL_GPL(pci_resources_share_page);
>>>> +
>>>>    static int __init pci_resource_alignment_sysfs_init(void)
>>>>    {
>>>>    	return bus_create_file(&pci_bus_type,
>>>> @@ -4859,6 +4891,9 @@ static int __init pci_setup(char *str)
>>>>    			} else if (!strncmp(str, "resource_alignment=", 19)) {
>>>>    				pci_set_resource_alignment_param(str + 19,
>>>>    							strlen(str + 19));
>>>> +			} else if (!strncmp(str, "resource_page_aligned=",
>>>> +				   22)) {
>>>> +				pci_resources_get_page_aligned(str + 22);
>>> Doesn't this seem rather redundant with the option right above it,
>>> resource_alignment=?  Why not modify that to support syntax where all
>>> devices get the same alignment?
>>>   
>>   
>> The kernel option will be used to do two things.
>>   
>> Firstly, the option can be used to enable all devices to be page aligned.
>>   
>> Secondly, we can use the option to disable it when the Kconfig option
>> mentioned above enable all devices to be page aligned by default.
>>   
>> We can easily modify this option "resource_alignment=" to do the first
>> thing. But I didn't find a proper way to modify it to do the second thing.
> You could allow an arch specified default which is overridden by
> specifying a resource_alignment= value.  Then you need a way to disable
> it, which you could simply do by making
> pci_set_resource_alignment_param() able to parse something like
> resource_alignment=off.

We just want to enforce the alignment of all MMIO BARs to be page
aligned in this patch. And both the arch specified default value and
pci_resources_page_aligned are something like *on/off* enforcing the
alignment of resources to be page aligned.

So I think it's better to add a parameter whose format is *on/off*.

If we reuse "resource_alignment=", we need an additional translation
from "resource_alignment="(format: [<order of align>@]...) to
pci_resources_page_aligned(format: true/false).

And "resource_alignment=" is always used to specify alignments of devices.
Would it be misunderstanding to add some syntax like
"resource_alignment=off"?

>>>>    			} else if (!strncmp(str, "ecrc=", 5)) {
>>>>    				pcie_ecrc_get_policy(str + 5);
>>>>    			} else if (!strncmp(str, "hpiosize=", 9)) {
>>>> diff --git a/drivers/pci/pci.h b/drivers/pci/pci.h
>>>> index d390fc1..b9b333d 100644
>>>> --- a/drivers/pci/pci.h
>>>> +++ b/drivers/pci/pci.h
>>>> @@ -312,11 +312,17 @@ static inline resource_size_t pci_resource_alignment(struct pci_dev *dev,
>>>>    #ifdef CONFIG_PCI_IOV
>>>>    	int resno = res - dev->resource;
>>>>    
>>>> -	if (resno >= PCI_IOV_RESOURCES && resno <= PCI_IOV_RESOURCE_END)
>>>> +	if (resno >= PCI_IOV_RESOURCES && resno <= PCI_IOV_RESOURCE_END) {
>>>> +		if (pci_resources_page_aligned && res->flags & IORESOURCE_MEM)
>>>> +			return PAGE_ALIGN(pci_sriov_resource_alignment(dev,
>>>> +					  resno));
>>>>    		return pci_sriov_resource_alignment(dev, resno);
>>>> +	}
>>>>    #endif
>>>>    	if (dev->class >> 8  == PCI_CLASS_BRIDGE_CARDBUS)
>>>>    		return pci_cardbus_resource_alignment(res);
>>>> +	if (pci_resources_page_aligned && res->flags & IORESOURCE_MEM)
>>>> +		return PAGE_ALIGN(resource_alignment(res));
>>>>    	return resource_alignment(res);
>>>>    }
>>>   
>>> Since we already have resource_alignment=, shouldn't we already have the
>>> code in place to re-align?
>>   
>> Yes, this code can do the re-aligning. But we can't reuse the code because
>> it re-align device's bars by changing their sizes, which can potentially
>> break
>> some drivers.
>>   
>> I'm thinking if we can use IORESOURCE_STARTALIGN for this. Thanks.
> Shouldn't we fix resource_alignment= then to make it behave in a more
> compatible way then?  resource_alignment=64k,resource_resize=off?
> Thanks,
>
> Alex
>

Good point! We can add something like "resource_resize=off" to
"resource_alignment=". Then we can reuse the re-aligning code. Thanks.

Regards,
Yongji Xie