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

From: Alex Williamson <[email-masked]>
Date: Mon, 04 Jan 2016 13:47:40 -0700

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.

> 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

>  #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?

Should this symbol be GPL-only?

> +
>  /* 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?  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.

>  
> 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)