linux-kernel - Re: [PATCH v3 5/8] iommu/vt-d: Implement def_domain

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Date:   Mon, 6 May 2019 16:25:48 +0100
From:   Tom Murphy <tmurphy@...sta.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     Christoph Hellwig <hch@...radead.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Joerg Roedel <joro@...tes.org>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Dmitry Safonov <dima@...sta.com>, linux-kernel@...r.kernel.org,
        iommu@...ts.linux-foundation.org, jacob.jun.pan@...el.com
Subject: Re: [PATCH v3 5/8] iommu/vt-d: Implement def_domain_type iommu ops entry

It looks like there is a bug in this code.

The behavior before this patch in __intel_map_single was that
iommu_no_mapping would call remove the attached si_domain for 32 bit
devices  (in the  dmar_remove_one_dev_info(dev) call in
iommu_no_mapping) and then allocate a new domain in
get_valid_domain_for_dev
old:
if (iommu_no_mapping(dev))
   return paddr;
domain = get_valid_domain_for_dev(dev);
if (!domain)
   return DMA_MAPPING_ERROR;

but in the new code we remove the attached si_domain but we WON'T
allocate a new domain and instead just return an error when we call
find_domain
new:
        if (iommu_no_mapping(dev))
                return paddr;

        domain = find_domain(dev);
        if (!domain)
                return DMA_MAPPING_ERROR;

This is a bug, right?

On Tue, Apr 30, 2019 at 3:18 AM Lu Baolu <baolu.lu@...ux.intel.com> wrote:
>
> Hi Christoph,
>
> On 4/30/19 4:03 AM, Christoph Hellwig wrote:
> >> @@ -3631,35 +3607,30 @@ static int iommu_no_mapping(struct device *dev)
> >>      if (iommu_dummy(dev))
> >>              return 1;
> >>
> >> -    if (!iommu_identity_mapping)
> >> -            return 0;
> >> -
> >
> > FYI, iommu_no_mapping has been refactored in for-next:
> >
> > https://git.kernel.org/pub/scm/linux/kernel/git/joro/iommu.git/commit/?h=x86/vt-d&id=48b2c937ea37a3bece0094b46450ed5267525289
>
> Oh, yes! Thanks for letting me know this. Will rebase the code.
>
> >
> >>      found = identity_mapping(dev);
> >>      if (found) {
> >> +            /*
> >> +             * If the device's dma_mask is less than the system's memory
> >> +             * size then this is not a candidate for identity mapping.
> >> +             */
> >> +            u64 dma_mask = *dev->dma_mask;
> >> +
> >> +            if (dev->coherent_dma_mask &&
> >> +                dev->coherent_dma_mask < dma_mask)
> >> +                    dma_mask = dev->coherent_dma_mask;
> >> +
> >> +            if (dma_mask < dma_get_required_mask(dev)) {
> >
> > I know this is mostly existing code moved around, but it really needs
> > some fixing.  For one dma_get_required_mask is supposed to return the
> > required to not bounce mask for the given device.  E.g. for a device
> > behind an iommu it should always just return 32-bit.  If you really
> > want to check vs system memory please call dma_direct_get_required_mask
> > without the dma_ops indirection.
> >
> > Second I don't even think we need to check the coherent_dma_mask,
> > dma_direct is pretty good at always finding memory even without
> > an iommu.
> >
> > Third this doesn't take take the bus_dma_mask into account.
> >
> > This probably should just be:
> >
> >               if (min(*dev->dma_mask, dev->bus_dma_mask) <
> >                   dma_direct_get_required_mask(dev)) {
>
> Agreed and will add this in the next version.
>
> Best regards,
> Lu Baolu