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

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Date:   Wed, 5 Oct 2022 10:53:50 +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 17:13, Niklas Schnelle wrote:
> On Tue, 2022-10-04 at 16:31 +0100, Robin Murphy wrote:
>> On 2022-10-04 16:12, Matthew Rosato wrote:
>>> 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().
>>
>> Perhaps, although the full change should be trivial enough that there's
>> probably just as much argument for doing the whole thing in its own
>> right for the sake of this cleanup. The main point is that
>> S390_IOMMU_PGSIZES is not incorrect as such, it's just not spelling out
>> the deliberate trick that it's achieving - everyone copied it from
>> intel-iommu, but since that got converted to the new interfaces the
>> original explanation is now gone. The only effect of "fixing" it in
>> isolation right now will be to make large VFIO mappings slower.
>>
>> Robin.
> 
> The patch changing to map_pages()/unmap_pages() is currently part of a
> larger series of improvements, some of which are less trivial. So I'm
> planning to send those as RFC first. Those include changing the
> spin_lock protected list to RCU so the map/unmap can paralellize
> better. Another one is atomic updates to the IOMMU tables to do away
> with locks in map/unmap. So I think pulling that whole
> series into this one isn't ideal. I could pull just the
> map_pages()/unmap_pages() change though.

Yeah, literally just updating the s390_iommu_{map,unmap} function 
prototypes and replacing "size" with "pgsize * count" within is all 
that's needed to clean up this hack properly. That can (and probably 
should) be completely independent of other improvements deeper down.

Thanks,
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,
> 
>