linux-kernel - Re: [PATCH v4 5/5] iommu/s390: Fix incorrect pgsize

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Date:   Tue, 4 Oct 2022 16:02:18 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Matthew Rosato <mjrosato@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>, iommu@...ts.linux.dev
Cc:     linux-s390@...r.kernel.org, borntraeger@...ux.ibm.com,
        hca@...ux.ibm.com, gor@...ux.ibm.com,
        gerald.schaefer@...ux.ibm.com, agordeev@...ux.ibm.com,
        svens@...ux.ibm.com, joro@...tes.org, will@...nel.org,
        jgg@...dia.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 5/5] iommu/s390: Fix incorrect pgsize_bitmap

On 2022-10-04 13:07, Niklas Schnelle wrote:
> The .pgsize_bitmap property of struct iommu_ops is not a page mask but
> rather has a bit set for each size of pages the IOMMU supports. As the
> comment correctly pointed out at this moment the code only support 4K
> pages so simply use SZ_4K here.

Unless it's already been done somewhere else, you'll want to switch over 
to the {map,unmap}_pages() interfaces as well to avoid taking a hit on 
efficiency here. The "page mask" thing was an old hack to trick the core 
API into making fewer map/unmap calls where the driver could map 
arbitrary numbers of pages at once anyway. The multi-page interfaces now 
do that more honestly and generally better (since they work for 
non-power-of-two sizes as well).

Robin.

> Reviewed-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Niklas Schnelle <schnelle@...ux.ibm.com>
> ---
>   drivers/iommu/s390-iommu.c | 9 +--------
>   1 file changed, 1 insertion(+), 8 deletions(-)
> 
> diff --git a/drivers/iommu/s390-iommu.c b/drivers/iommu/s390-iommu.c
> index 94c444b909bd..6bf23e7830a2 100644
> --- a/drivers/iommu/s390-iommu.c
> +++ b/drivers/iommu/s390-iommu.c
> @@ -12,13 +12,6 @@
>   #include <linux/sizes.h>
>   #include <asm/pci_dma.h>
>   
> -/*
> - * Physically contiguous memory regions can be mapped with 4 KiB alignment,
> - * we allow all page sizes that are an order of 4KiB (no special large page
> - * support so far).
> - */
> -#define S390_IOMMU_PGSIZES	(~0xFFFUL)
> -
>   static const struct iommu_ops s390_iommu_ops;
>   
>   struct s390_domain {
> @@ -350,7 +343,7 @@ static const struct iommu_ops s390_iommu_ops = {
>   	.probe_device = s390_iommu_probe_device,
>   	.release_device = s390_iommu_release_device,
>   	.device_group = generic_device_group,
> -	.pgsize_bitmap = S390_IOMMU_PGSIZES,
> +	.pgsize_bitmap = SZ_4K,
>   	.get_resv_regions = s390_iommu_get_resv_regions,
>   	.default_domain_ops = &(const struct iommu_domain_ops) {
>   		.attach_dev	= s390_iommu_attach_device,