Re: [PATCH v2 3/6] x86/iommu: introduce a rangeset to perform hwdom IOMMU setup

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.12.2023 10:43, Roger Pau Monne wrote:
> This change just introduces the boilerplate code in order to use a rangeset
> when setting up the hardware domain IOMMU mappings.  The rangeset is never
> populated in this patch, so it's a non-functional change as far as the 
> mappings
> the domain gets established.
> 
> Note there's a change for HVM domains (ie: PVH dom0) that will get switched to
> create the p2m mappings using map_mmio_regions() instead of
> p2m_add_identity_entry(), so that ranges can be mapped with a single function
> call if possible.  Note that the interface of map_mmio_regions() doesn't allow
> creating read-only mappings, but so far there are no such mappings created for
> PVH dom0 in arch_iommu_hwdom_init().

I don't understand this paragraph: The rangeset remains empty, so nothing is
changing right here. DYM there is going to be such a change as a result of
this patch, but in a later part of this series?

> --- a/xen/drivers/passthrough/x86/iommu.c
> +++ b/xen/drivers/passthrough/x86/iommu.c
> @@ -370,10 +370,77 @@ static unsigned int __hwdom_init hwdom_iommu_map(const 
> struct domain *d,
>      return perms;
>  }
>  
> +struct map_data {
> +    struct domain *d;
> +    unsigned int flush_flags;
> +    bool ro;
> +};
> +
> +static int __hwdom_init cf_check identity_map(unsigned long s, unsigned long 
> e,
> +                                              void *data)
> +{
> +    struct map_data *info = data;
> +    struct domain *d = info->d;
> +    long rc;
> +
> +    if ( iommu_verbose )
> +        printk(XENLOG_INFO " [%010lx, %010lx] R%c\n",
> +               s, e, info->ro ? 'O' : 'W');
> +
> +    if ( paging_mode_translate(d) )
> +    {
> +        if ( info->ro )
> +        {
> +            ASSERT_UNREACHABLE();
> +            return 0;
> +        }
> +        while ( (rc = map_mmio_regions(d, _gfn(s), e - s + 1, _mfn(s))) > 0 )
> +        {
> +            s += rc;
> +            process_pending_softirqs();
> +        }
> +    }
> +    else
> +    {
> +        const unsigned int perms = IOMMUF_readable | IOMMUF_preempt |
> +                                   (info->ro ? 0 : IOMMUF_writable);
> +
> +        if ( info->ro && !iomem_access_permitted(d, s, e) )

How is r/o-ness related to iomem_access_permitted()? The present callers
are such that there is a connection, but that's invisible here. I guess
either the field wants to change name (maybe mmio_ro or ro_mmio or even
just mmio), or there wants to be a comment.

> +        {
> +            /*
> +             * Should be more fine grained in order to not map the forbidden
> +             * frame instead of rejecting the region as a whole, but it's 
> only
> +             * for read-only MMIO regions, which are very limited.
> +             */

How certain are you/we that no two adjacent ones may appear, with
different permissions granted to Dom0?

> +            printk(XENLOG_DEBUG
> +                   "IOMMU read-only mapping of region [%lx, %lx] 
> forbidden\n",
> +                   s, e);
> +            return 0;
> +        }
> +        while ( (rc = iommu_map(d, _dfn(s), _mfn(s), e - s + 1,
> +                                perms, &info->flush_flags)) > 0 )
> +        {
> +            s += rc;
> +            process_pending_softirqs();
> +        }
> +    }
> +    ASSERT(rc <= 0);
> +    if ( rc )
> +        printk(XENLOG_WARNING
> +               "IOMMU identity mapping of [%lx, %lx] failed: %ld\n",
> +               s, e, rc);
> +
> +    /* Ignore errors and attempt to map the remaining regions. */
> +    return 0;
> +}
> +
>  void __hwdom_init arch_iommu_hwdom_init(struct domain *d)
>  {
>      unsigned long i, top, max_pfn, start, count;
>      unsigned int flush_flags = 0, start_perms = 0;
> +    struct rangeset *map;
> +    struct map_data map_data = { .d = d };
> +    int rc;
>  
>      BUG_ON(!is_hardware_domain(d));
>  
> @@ -397,6 +464,10 @@ void __hwdom_init arch_iommu_hwdom_init(struct domain *d)
>      if ( iommu_hwdom_passthrough )
>          return;
>  
> +    map = rangeset_new(NULL, NULL, 0);
> +    if ( !map )
> +        panic("IOMMU init: unable to allocate rangeset\n");
> +
>      max_pfn = (GB(4) >> PAGE_SHIFT) - 1;
>      top = max(max_pdx, pfn_to_pdx(max_pfn) + 1);
>  
> @@ -451,6 +522,24 @@ void __hwdom_init arch_iommu_hwdom_init(struct domain *d)
>              goto commit;
>      }
>  
> +    if ( iommu_verbose )
> +        printk(XENLOG_INFO "d%u: identity mappings for IOMMU:\n",
> +               d->domain_id);

%pd: ?

> +    rc = rangeset_report_ranges(map, 0, ~0UL, identity_map, &map_data);
> +    if ( rc )
> +        panic("IOMMU unable to create mappings: %d\n", rc);
> +    if ( is_pv_domain(d) )
> +    {
> +        map_data.ro = true;
> +        rc = rangeset_report_ranges(mmio_ro_ranges, 0, ~0UL, identity_map,
> +                                    &map_data);
> +        if ( rc )
> +            panic("IOMMU unable to create read-only mappings: %d\n", rc);
> +    }
> +
> +    rangeset_destroy(map);

This could move up, couldn't it?

>      /* Use if to avoid compiler warning */
>      if ( iommu_iotlb_flush_all(d, flush_flags) )

Don't you need to fold map.flush_flags into flush_flags ahead of this call?
Or can the variable perhaps go away altogether, being replaced by the struct
field?

Jan

>          return;