Re: [PATCH v6 3/3] PCI: s390: Enable HAVE_PCI_MMAP on s390 and restrict mmap() of resources to mappable BARs

[Niklas Schnelle <schnelle@linux.ibm.com>]

On Fri, 2025-02-14 at 14:10 +0100, Niklas Schnelle wrote:
> So far s390 does not select HAVE_PCI_MMAP. This is partly because access
> to mapped PCI resources requires special PCI load/store instructions and
> prior to commit 71ba41c9b1d9 ("s390/pci: provide support for MIO
> instructions") even required use of special syscalls. This really isn't
> a showstopper though and in fact lack of HAVE_PCI_MMAP has previously
> caused extra work when testing and debugging PCI devices and drivers.
> 
> Another issue when looking at HAVE_PCI_MMAP however comes from the
> virtual ISM devices. These present 256 TiB BARs which really can't be
> accessed via a mapping to user-space.
> 
> Now, the newly added pdev->non_mappable_bars flag provides a way to
> exclude devices whose BARs can't be mapped to user-space including the
> s390 ISM device. So honor this flag also in the mmap() paths protected
> by HAVE_PCI_MMMAP and with the ISM device thus excluded enable
> HAVE_PCI_MMAP for s390.
> 
> Note that most distributions enable CONFIG_IO_STRICT_DEVMEM=y and
> require unbinding drivers before resources can be mapped. This makes it
> extremely unlikely that any existing programs on s390 will now suddenly
> fail after succeeding to mmap() resources and then trying to access the
> mapping without use of the special PCI instructions.
> 
> Link: https://lore.kernel.org/lkml/20250212132808.08dcf03c.alex.williamson@redhat.com/
> Suggested-by: Alex Williamson <alex.williamson@redhat.com>
> Signed-off-by: Niklas Schnelle <schnelle@linux.ibm.com>
> ---
>  arch/s390/include/asm/pci.h | 4 ++++
>  drivers/pci/pci-sysfs.c     | 4 ++++
>  drivers/pci/proc.c          | 4 ++++
>  3 files changed, 12 insertions(+)
> 
> diff --git a/arch/s390/include/asm/pci.h b/arch/s390/include/asm/pci.h
> index 474e1f8d1d3c2fc5685b459cc68b67ac651ea3e9..518dd71a78c83c74dc7b29778e299d5c8cabcc59 100644
> --- a/arch/s390/include/asm/pci.h
> +++ b/arch/s390/include/asm/pci.h
> @@ -11,6 +11,10 @@
>  #include <asm/pci_insn.h>
>  #include <asm/sclp.h>
>  
> +#define HAVE_PCI_MMAP			1
> +#define ARCH_GENERIC_PCI_MMAP_RESOURCE	1
> +#define arch_can_pci_mmap_wc()		1
> +
>  #define PCIBIOS_MIN_IO		0x1000
>  #define PCIBIOS_MIN_MEM		0x10000000
>  
> diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> index b46ce1a2c5542cdea0a3f9df324434fdb7e8a4d2..7373eca0a4943bf896b4a177124e0d4572baec2b 100644
> --- a/drivers/pci/pci-sysfs.c
> +++ b/drivers/pci/pci-sysfs.c
> @@ -1257,6 +1257,10 @@ static int pci_create_resource_files(struct pci_dev *pdev)
>  	int i;
>  	int retval;
>  
> +	/* Skip devices with non-mappable BARs */
> +	if (pdev->non_mappable_bars)
> +		return 0;
> +
>  	/* Expose the PCI resources from this device as files */
>  	for (i = 0; i < PCI_STD_NUM_BARS; i++) {
>  
> diff --git a/drivers/pci/proc.c b/drivers/pci/proc.c
> index f967709082d654a101039091b5493b2dec5f57b4..9348a0fb808477ca9be80a8b88bbc036565bc411 100644
> --- a/drivers/pci/proc.c
> +++ b/drivers/pci/proc.c
> @@ -251,6 +251,10 @@ static int proc_bus_pci_mmap(struct file *file, struct vm_area_struct *vma)
>  	    security_locked_down(LOCKDOWN_PCI_ACCESS))
>  		return -EPERM;
>  
> +	/* Skip devices with non-mappable BARs */
> +	if (dev->non_mappable_bars)
> +		return -EINVAL;
> +
>  	if (fpriv->mmap_state == pci_mmap_io) {
>  		if (!arch_can_pci_mmap_io())
>  			return -EINVAL;
> 

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