Subject: Re: [RFC PATCH v2 1/3] PCI: Add support for enforcing all MMIO BARs to be page aligned

From: Yongji Xie <[email-masked]>
Date: Tue, 5 Jan 2016 19:04:40 +0800

On 2016/1/5 4:47, Alex Williamson wrote:
> On Thu, 2015-12-31 at 16:50 +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 alignments 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".
> Shouldn't this somehow be associated with the realloc option?  I don't
> think PCI code will attempt to reprogram anything unless it needs to
> otherwise.

So you mean we need to ignore firmware setup and force re-assigning all
resources if we want to use the option "pci=resource_page_aligned=on"?

>> For the default value of this parameter, we think it should be
>> arch-independent, so we add a macro PCI_RESOURCE_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.
>>
>> Signed-off-by: Yongji Xie <[email-masked]>
>> ---
>>   Documentation/kernel-parameters.txt |    4 ++++
>>   arch/powerpc/include/asm/pci.h      |   11 +++++++++++
>>   drivers/pci/pci.c                   |   17 +++++++++++++++++
>>   drivers/pci/pci.h                   |    7 ++++++-
>>   include/linux/pci.h                 |    2 ++
>>   5 files changed, 40 insertions(+), 1 deletion(-)
>>
>> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
>> index 742f69d..a53aaee 100644
>> --- a/Documentation/kernel-parameters.txt
>> +++ b/Documentation/kernel-parameters.txt
>> @@ -2857,6 +2857,10 @@ 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.
>> +				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..27bff59 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 bootcmd
>> + * "pci=resource_page_aligned=off" to disable it.
>> + */
>> +#define PCI_ENABLE_RESOURCE_PAGE_ALIGNED
>> +
>> +#endif
> This should be done with something like HAVE_PCI_DEFAULT_RESOURCE_PAGE_
> ALIGNED in arch/powerpc/include/asm

OK, I will fix it in next version.

>>   #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..9f14ba5 100644
>> --- a/drivers/pci/pci.c
>> +++ b/drivers/pci/pci.c
>> @@ -99,6 +99,13 @@ u8 pci_cache_line_size;
>>    */
>>   unsigned int pcibios_max_latency = 255;
>>   
>> +#ifdef PCI_ENABLE_RESOURCE_PAGE_ALIGNED
>> +bool pci_resource_page_aligned = true;
>> +#else
>> +bool pci_resource_page_aligned;
>> +#endif
>> +EXPORT_SYMBOL(pci_resource_page_aligned);
> Couldn't this be done in a single line with IS_ENABLED() macro?

I'm not sure whether IS_ENABLED() macro should be used there because
it is always used for CONFIG_ options.

> Should this symbol be GPL-only?

Yes, it will be fixed in next version.

>> +
>>   /* If set, the PCIe ARI capability will not be used. */
>>   static bool pcie_ari_disabled;
>>   
>> @@ -4746,6 +4753,14 @@ 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_resource_get_page_aligned(char *str)
>> +{
>> +	if (!strncmp(str, "off", 3))
>> +		pci_resource_page_aligned = false;
>> +	else if (!strncmp(str, "on", 2))
>> +		pci_resource_page_aligned = true;
>> +}
>> +
>>   static int __init pci_resource_alignment_sysfs_init(void)
>>   {
>>   	return bus_create_file(&pci_bus_type,
>> @@ -4859,6 +4874,8 @@ 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_resource_get_page_aligned(str + 22);
>>   			} 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..e16e48c 100644
>> --- a/drivers/pci/pci.h
>> +++ b/drivers/pci/pci.h
>> @@ -312,11 +312,16 @@ 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_resource_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_resource_page_aligned && res->flags & IORESOURCE_MEM)
>> +		return PAGE_ALIGN(resource_alignment(res));
>>   	return resource_alignment(res);
>>   }
> Why are we calling resource_alignment() and then doing another
> alignment on top of that?  Shouldn't this alignment be done there?

We doing the alignment in pci_resource_alignment() because this is a
PCI specific feature. And I don't think pci_resource_page_aligned should
be used in resource_alignment().

> I still don't really understand if this is enough to produce a
> reallocation if the alignment is not sufficient, vfio would need to
> know if pci_resource_page_aligned has any loopholes.

Do you mean doing this in pci_resource_alignment() may not
be enough to make sure the alignments of all PCI resources are page
aligned? Thanks.

Regards
Yongji Xie

>>   
>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>> index 6ae25aa..0ca57f1 100644
>> --- a/include/linux/pci.h
>> +++ b/include/linux/pci.h
>> @@ -1517,6 +1517,8 @@ static inline int pci_get_new_domain_nr(void) { return -ENOSYS; }
>>   
>>   #include
>>   
>> +extern bool pci_resource_page_aligned;
>> +
>>   /* these helpers provide future and backwards compatibility
>>    * for accessing popular PCI BAR info */
>>   #define pci_resource_start(dev, bar)	((dev)->resource[(bar)].start)