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

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Message-ID: <b3c6fd2b-74d4-91d7-dc53-ba526c41b067@linux.ibm.com>
Date:   Tue, 4 Oct 2022 11:12:41 -0400
From:   Matthew Rosato <mjrosato@...ux.ibm.com>
To:     Robin Murphy <robin.murphy@....com>,
        Niklas Schnelle <schnelle@...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 10/4/22 11:02 AM, Robin Murphy wrote:
> 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).

Thanks for the heads up -- Niklas has some additional series coming soon as described here:

https://lore.kernel.org/linux-iommu/a10424adbe01a0fd40372cbd0736d11e517951a1.camel@linux.ibm.com/

So implementing the _pages() interfaces is soon up on the roadmap.  But given what you say I wonder if this patch should just wait until the series that implements {map,unmap}_pages().
> 
> 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,