linux-kernel - Re: [PATCH v2 1/3] iommu/s390: Fix duplicate domain attachments

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Message-ID: <a4005a4ae804f852656ff2a6f2f994958b0b02e3.camel@linux.ibm.com>
Date:   Wed, 28 Sep 2022 10:58:22 +0200
From:   Niklas Schnelle <schnelle@...ux.ibm.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Matthew Rosato <mjrosato@...ux.ibm.com>,
        Pierre Morel <pmorel@...ux.ibm.com>, iommu@...ts.linux.dev,
        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,
        robin.murphy@....com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] iommu/s390: Fix duplicate domain attachments

On Tue, 2022-09-27 at 13:56 -0300, Jason Gunthorpe wrote:
> On Tue, Sep 27, 2022 at 06:33:48PM +0200, Niklas Schnelle wrote:
>  
> > Not sure what the non-MSI reservation is for? It does seem like x86_64
> > also uses this for quite large ranges.
> 
> There are lots of things that are unsuitable for DMA on x86 platforms,
> unfortunately.. But yeah, I'm not sure either.
> 
> > This is because I'm getting a map request for an IOVA in the reserved
> > region.
> 
> How come? iova_reserve_iommu_regions() reads the reserved regions and
> loads them as reserved into the iovad which should cause
> iommu_dma_alloc_iova() and alloc_iova_fast() to not return values in
> those ranges.
> 
> It all looks like it is supposed to work
> 
> Did something go wrong in the initialization order perhaps?
> 
> Jason

It was of course a classic off-by-one, the table size is a number of
entries but geometry.aperture_end seems to be the largest allowed IOVA.
So we need:

s390_domain->domain.geometry.force_aperture = true;
s390_domain->domain.geometry.aperture_start = 0;
s390_domain->domain.geometry.aperture_end = ZPCI_TABLE_SIZE_RT - 1;

Otherwise the first IOVA allocated is ZPCI_TABLE_SIZE_RT itself.
Similarly we need the second reserved region if (zdev->end_dma <
ZPCI_TABLE_SIZE_RT - 1). In your patch I think you had the
MAX_DMA_TABLE_ADDR name right but would have also calculated the number
of entries.

On the other hand with the dma-iommu.c conversion it no longer makes
sense to lower zdev->end_dma artificially, so at least on current
machine LPARs we would end up with just a lower reserved region
0x0000000000000000 to 0x00000000ffffffff and can use IOVAs up to
aperture_end.