Re: [RFC PATCH v4 4/7] PCI: Modify resource_alignment to support multiple devices

[Alex Williamson]

On Thu, 17 Mar 2016 19:28:34 +0800
Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx> wrote:

> On 2016/3/17 0:30, Alex Williamson wrote:
> > On Mon,  7 Mar 2016 15:48:35 +0800
> > Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx> 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 modifies
> >> resource_alignment to support syntax where multiple
> >> devices get the same alignment. So we can use something
> >> like "pci=resource_alignment=*:*:*.*:noresize" 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.
> >>
> >> Signed-off-by: Yongji Xie <xyjxie@xxxxxxxxxxxxxxxxxx>
> >> ---
> >>   Documentation/kernel-parameters.txt |    2 +
> >>   drivers/pci/pci.c                   |   90 ++++++++++++++++++++++++++++++-----
> >>   include/linux/pci.h                 |    4 ++
> >>   3 files changed, 85 insertions(+), 11 deletions(-)
> >>
> >> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> >> index 8028631..74b38ab 100644
> >> --- a/Documentation/kernel-parameters.txt
> >> +++ b/Documentation/kernel-parameters.txt
> >> @@ -2918,6 +2918,8 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
> >>   				aligned memory resources.
> >>   				If <order of align> is not specified,
> >>   				PAGE_SIZE is used as alignment.
> >> +				<domain>, <bus>, <slot> and <func> can be set to
> >> +				"*" which means match all values.  
> > I don't see anywhere that you're automatically enabling this for your
> > platform, so presumably you're expecting users to determine on their
> > own that they have a performance problem and hoping that they'll figure
> > out that they need to use this option to resolve it.  The first irate
> > question you'll get back is why doesn't this happen automatically?  
> 
> Actually, I just want to make the code simple. Maybe it is
> not a good idea to let users enable this option manually.
> I will try to fix it like that:

That's not entirely what I meant, I think having a way for a user to
enable it is a good thing, but maybe there need to be cases where it's
enabled automatically.  With 4k pages, this is often not an issue since
the PCI spec recommends 4k or 8k alignment for resources, but that
doesn't preclude an unusual device where a user might want to enable it
anyway.  At 64k page size, problems become more common, so we need to
think about either enabling it automatically or somehow making it more
apparent to the user that the option is available for this purpose.

> 
> diff --git a/arch/powerpc/include/asm/pci.h b/arch/powerpc/include/asm/pci.h
> index 6f8065a..6659752 100644
> --- a/arch/powerpc/include/asm/pci.h
> +++ b/arch/powerpc/include/asm/pci.h
> @@ -30,6 +30,8 @@
>   #define PCIBIOS_MIN_IO         0x1000
>   #define PCIBIOS_MIN_MEM                0x10000000
> 
> +#define PCIBIOS_MIN_ALIGNMENT  PAGE_SIZE
> +
>   struct pci_dev;
> 
>   /* Values for the `which' argument to sys_pciconfig_iobase syscall.  */
> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> index dadd28a..9f644e4 100644
> --- a/drivers/pci/pci.c
> +++ b/drivers/pci/pci.c
> @@ -4593,6 +4593,8 @@ EXPORT_SYMBOL_GPL(pci_ignore_hotplug);
>   static char resource_alignment_param[RESOURCE_ALIGNMENT_PARAM_SIZE] = {0};
>   static DEFINE_SPINLOCK(resource_alignment_lock);
> 
> +#define DISABLE_ARCH_ALIGNMENT -1
> +#define DEFAULT_ALIGNMENT      -2
>   /**
>    * pci_specified_resource_alignment - get resource alignment specified 
> by user.
>    * @dev: the PCI device to get
> @@ -4609,6 +4611,9 @@ static resource_size_t 
> pci_specified_resource_alignment(struct pci_dev *dev,
>          char *p;
>          bool invalid = false;
> 
> +#ifdef PCIBIOS_MIN_ALIGNMENT
> +       align = PCIBIOS_MIN_ALIGNMENT;
> +#endif
>          spin_lock(&resource_alignment_lock);
>          p = resource_alignment_param;
>          while (*p) {
> @@ -4617,7 +4622,7 @@ static resource_size_t 
> pci_specified_resource_alignment(struct pci_dev *dev,
>                                                          p[count] == '@') {
>                          p += count + 1;
>                  } else {
> -                       align_order = -1;
> +                       align_order = DEFAULT_ALIGNMENT;
>                  }
>                  if (p[0] == '*' && p[1] == ':') {
>                          seg = -1;
> @@ -4673,8 +4678,10 @@ static resource_size_t 
> pci_specified_resource_alignment(struct pci_dev *dev,
>                          (bus == dev->bus->number || bus == -1) &&
>                          (slot == PCI_SLOT(dev->devfn) || slot == -1) &&
>                          (func == PCI_FUNC(dev->devfn) || func == -1)) {
> -                       if (align_order == -1)
> +                       if (align_order == DEFAULT_ALIGNMENT)
>                                  align = PAGE_SIZE;
> +                       else if (align_order == DISABLE_ARCH_ALIGNMENT)
> +                               align = 0;
>                          else
>                                  align = 1 << align_order;
>                          if (!pci_resources_page_aligned &&
> >>   				PCI-PCI bridge can be specified, if resource
> >>   				windows need to be expanded.
> >>   				noresize: Don't change the resources' sizes when
> >> diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> >> index 760cce5..44ab59f 100644
> >> --- a/drivers/pci/pci.c
> >> +++ b/drivers/pci/pci.c
> >> @@ -102,6 +102,8 @@ unsigned int pcibios_max_latency = 255;
> >>   /* If set, the PCIe ARI capability will not be used. */
> >>   static bool pcie_ari_disabled;
> >>   
> >> +bool pci_resources_page_aligned;
> >> +
> >>   /**
> >>    * pci_bus_max_busnr - returns maximum PCI bus number of given bus' children
> >>    * @bus: pointer to PCI bus structure to search
> >> @@ -4604,6 +4606,7 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
> >>   	int seg, bus, slot, func, align_order, count;
> >>   	resource_size_t align = 0;
> >>   	char *p;
> >> +	bool invalid = false;
> >>   
> >>   	spin_lock(&resource_alignment_lock);
> >>   	p = resource_alignment_param;
> >> @@ -4615,16 +4618,49 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
> >>   		} else {
> >>   			align_order = -1;
> >>   		}
> >> -		if (sscanf(p, "%x:%x:%x.%x%n",
> >> -			&seg, &bus, &slot, &func, &count) != 4) {
> >> +		if (p[0] == '*' && p[1] == ':') {
> >> +			seg = -1;
> >> +			count = 1;
> >> +		} else if (sscanf(p, "%x%n", &seg, &count) != 1 ||
> >> +				p[count] != ':') {
> >> +			invalid = true;
> >> +			break;
> >> +		}
> >> +		p += count + 1;
> >> +		if (*p == '*') {
> >> +			bus = -1;
> >> +			count = 1;
> >> +		} else if (sscanf(p, "%x%n", &bus, &count) != 1) {
> >> +			invalid = true;
> >> +			break;
> >> +		}
> >> +		p += count;
> >> +		if (*p == '.') {
> >> +			slot = bus;
> >> +			bus = seg;
> >>   			seg = 0;
> >> -			if (sscanf(p, "%x:%x.%x%n",
> >> -					&bus, &slot, &func, &count) != 3) {
> >> -				/* Invalid format */
> >> -				printk(KERN_ERR "PCI: Can't parse resource_alignment parameter: %s\n",
> >> -					p);
> >> +			p++;
> >> +		} else if (*p == ':') {
> >> +			p++;
> >> +			if (p[0] == '*' && p[1] == '.') {
> >> +				slot = -1;
> >> +				count = 1;
> >> +			} else if (sscanf(p, "%x%n", &slot, &count) != 1 ||
> >> +					p[count] != '.') {
> >> +				invalid = true;
> >>   				break;
> >>   			}
> >> +			p += count + 1;
> >> +		} else {
> >> +			invalid = true;
> >> +			break;
> >> +		}
> >> +		if (*p == '*') {
> >> +			func = -1;
> >> +			count = 1;
> >> +		} else if (sscanf(p, "%x%n", &func, &count) != 1) {
> >> +			invalid = true;
> >> +			break;
> >>   		}
> >>   		p += count;
> >>   		if (!strncmp(p, ":noresize", 9)) {
> >> @@ -4632,23 +4668,34 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
> >>   			p += 9;
> >>   		} else
> >>   			*resize = true;
> >> -		if (seg == pci_domain_nr(dev->bus) &&
> >> -			bus == dev->bus->number &&
> >> -			slot == PCI_SLOT(dev->devfn) &&
> >> -			func == PCI_FUNC(dev->devfn)) {
> >> +		if ((seg == pci_domain_nr(dev->bus) || seg == -1) &&
> >> +			(bus == dev->bus->number || bus == -1) &&
> >> +			(slot == PCI_SLOT(dev->devfn) || slot == -1) &&
> >> +			(func == PCI_FUNC(dev->devfn) || func == -1)) {
> >>   			if (align_order == -1)
> >>   				align = PAGE_SIZE;
> >>   			else
> >>   				align = 1 << align_order;
> >> +			if (!pci_resources_page_aligned &&
> >> +				(align >= PAGE_SIZE &&
> >> +				seg == -1 && bus == -1 &&
> >> +				slot == -1 && func == -1))
> >> +				pci_resources_page_aligned = true;
> >>   			/* Found */
> >>   			break;
> >>   		}
> >>   		if (*p != ';' && *p != ',') {
> >>   			/* End of param or invalid format */
> >> +			invalid = true;
> >>   			break;
> >>   		}
> >>   		p++;
> >>   	}
> >> +	if (invalid == true) {
> >> +		/* Invalid format */
> >> +		printk(KERN_ERR "PCI: Can't parse resource_alignment parameter:%s\n",
> >> +				p);
> >> +	}
> >>   	spin_unlock(&resource_alignment_lock);
> >>   	return align;
> >>   }
> >> @@ -4769,6 +4816,27 @@ static int __init pci_resource_alignment_sysfs_init(void)
> >>   }
> >>   late_initcall(pci_resource_alignment_sysfs_init);
> >>   
> >> +/*
> >> + * 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);  
> > This should be a separate patch, there's no mention of this part of the
> > change at all in the commitlog.  Also, pci_resource_page_aligned is
> > only set if we use the magic wildcards to set alignment for all
> > devices, it's not set if we use a specific seg:bus:dev.fn.  That's
> > not consistent.  Can't we make use of the STARTALIGN flag for this or
> > did that get removed when resources were assigned?  
> 
> I only set pci_resource_page_aligned when using magic wildcards
> because I must make sure pci_resources_share_page() can return
> correctly when a hotplug event happens.
> 
> For example, there is only one half-page resource in the system.
> And we use a specific seg:bus:dev.fn to force it to be page aligned.
> So we set pci_resource_page_aligned because all devices are page
> aligned now. It will return false when we call
> pci_resources_share_page() for this resource. But this return value
> will be not correct if we hot add a new device which also have
> a half-page resource which will share one page with the previous
> resource.

That's a good point, it should be documented in a comment.

> > The test itself is not entirely reassuring, I'd like some positive
> > indication that the device has been aligned, not simply that it should
> > have been and the start alignment appears that it might have happened.
> > Apparently you don't trust pci_resources_page_aligned either since you
> > still test the start alignment.  
> 
> Yes, I don't trust pci_resources_page_aligned. Because
> resource_alignment will not work in some case, e.g.
> PCI_PROBE_ONLY or IORESOURCE_PCI_FIXED is set.
> 
> But I think *start* of the resource can give the positive
> indication. The resource should not share page with
> others if the start of the resource and its sibling are both
> page aligned.

If you can't trust pci_resource_page_aligned then the alignment of
start seems like it only indicates that it's likely aligned, not that
it is aligned.

> >> +
> >>   static void pci_no_domains(void)
> >>   {
> >>   #ifdef CONFIG_PCI_DOMAINS
> >> diff --git a/include/linux/pci.h b/include/linux/pci.h
> >> index 2771625..064a1b6 100644
> >> --- a/include/linux/pci.h
> >> +++ b/include/linux/pci.h
> >> @@ -1511,6 +1511,10 @@ static inline int pci_get_new_domain_nr(void) { return -ENOSYS; }
> >>   	 (pci_resource_end((dev), (bar)) -		\
> >>   	  pci_resource_start((dev), (bar)) + 1))
> >>   
> >> +extern bool pci_resources_page_aligned;  
> > I'm not seeing why this shouldn't have been static since you're
> > providing a function that tests it, there shouldn't really be any
> > external consumers.  
> 
> I made a mistake here. I will fix it in next version.
> 
> Thanks,
> Yongji Xie.
>