linux-kernel - Re: [PATCH v2 1/2] iommu/virtio: Make use of ops->iotlb_sync

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Date:   Fri, 22 Sep 2023 14:13:18 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     Jason Gunthorpe <jgg@...pe.ca>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Cc:     Niklas Schnelle <schnelle@...ux.ibm.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        virtualization@...ts.linux-foundation.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] iommu/virtio: Make use of ops->iotlb_sync_map

On 22/09/2023 1:41 pm, Jason Gunthorpe wrote:
> On Fri, Sep 22, 2023 at 08:57:19AM +0100, Jean-Philippe Brucker wrote:
>>>> They're not strictly equivalent: this check works around a temporary issue
>>>> with the IOMMU core, which calls map/unmap before the domain is
>>>> finalized.
>>>
>>> Where? The above points to iommu_create_device_direct_mappings() but
>>> it doesn't because the pgsize_bitmap == 0:
>>
>> __iommu_domain_alloc() sets pgsize_bitmap in this case:
>>
>>          /*
>>           * If not already set, assume all sizes by default; the driver
>>           * may override this later
>>           */
>>          if (!domain->pgsize_bitmap)
>>                  domain->pgsize_bitmap = bus->iommu_ops->pgsize_bitmap;
> 
> Dirver's shouldn't do that.
> 
> The core code was fixed to try again with mapping reserved regions to
> support these kinds of drivers.

This is still the "normal" code path, really; I think it's only AMD that 
started initialising the domain bitmap "early" and warranted making it 
conditional. However we *do* ultimately want all the drivers to do the 
same, so we can get rid of ops->pgsize_bitmap, because it's already 
pretty redundant and meaningless in the face of per-domain pagetable 
formats.

Thanks,
Robin.