Re: [PATCH v2 05/18] IOMMU: have iommu_{,un}map() split requests into largest possible chunks

["Roger Pau Monné" <roger.pau@citrix.com>]

On Fri, Sep 24, 2021 at 11:45:57AM +0200, Jan Beulich wrote:
> Introduce a helper function to determine the largest possible mapping
> that allows covering a request (or the next part of it that is left to
> be processed).
> 
> In order to not add yet more recurring dfn_add() / mfn_add() to the two
> callers of the new helper, also introduce local variables holding the
> values presently operated on.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> 
> --- a/xen/drivers/passthrough/iommu.c
> +++ b/xen/drivers/passthrough/iommu.c
> @@ -260,12 +260,38 @@ void iommu_domain_destroy(struct domain
>      arch_iommu_domain_destroy(d);
>  }
>  
> -int iommu_map(struct domain *d, dfn_t dfn, mfn_t mfn,
> +static unsigned int mapping_order(const struct domain_iommu *hd,
> +                                  dfn_t dfn, mfn_t mfn, unsigned long nr)
> +{
> +    unsigned long res = dfn_x(dfn) | mfn_x(mfn);
> +    unsigned long sizes = hd->platform_ops->page_sizes;
> +    unsigned int bit = find_first_set_bit(sizes), order = 0;
> +
> +    ASSERT(bit == PAGE_SHIFT);
> +
> +    while ( (sizes = (sizes >> bit) & ~1) )
> +    {
> +        unsigned long mask;
> +
> +        bit = find_first_set_bit(sizes);
> +        mask = (1UL << bit) - 1;
> +        if ( nr <= mask || (res & mask) )
> +            break;
> +        order += bit;
> +        nr >>= bit;
> +        res >>= bit;
> +    }
> +
> +    return order;
> +}

This looks like it could be used in other places, I would at least
consider using it in pvh_populate_memory_range where we also need to
figure out the maximum order given an address and a number of pages.

Do you think you could place it in a more generic file and also use
more generic parameters (ie: unsigned long gfn and mfn)?

> +
> +int iommu_map(struct domain *d, dfn_t dfn0, mfn_t mfn0,
>                unsigned long page_count, unsigned int flags,
>                unsigned int *flush_flags)
>  {
>      const struct domain_iommu *hd = dom_iommu(d);
>      unsigned long i;
> +    unsigned int order;
>      int rc = 0;
>  
>      if ( !is_iommu_enabled(d) )
> @@ -273,10 +299,16 @@ int iommu_map(struct domain *d, dfn_t df
>  
>      ASSERT(!IOMMUF_order(flags));
>  
> -    for ( i = 0; i < page_count; i++ )
> +    for ( i = 0; i < page_count; i += 1UL << order )
>      {
> -        rc = iommu_call(hd->platform_ops, map_page, d, dfn_add(dfn, i),
> -                        mfn_add(mfn, i), flags, flush_flags);
> +        dfn_t dfn = dfn_add(dfn0, i);
> +        mfn_t mfn = mfn_add(mfn0, i);
> +        unsigned long j;
> +
> +        order = mapping_order(hd, dfn, mfn, page_count - i);
> +
> +        rc = iommu_call(hd->platform_ops, map_page, d, dfn, mfn,
> +                        flags | IOMMUF_order(order), flush_flags);
>  
>          if ( likely(!rc) )
>              continue;
> @@ -284,14 +316,18 @@ int iommu_map(struct domain *d, dfn_t df
>          if ( !d->is_shutting_down && printk_ratelimit() )
>              printk(XENLOG_ERR
>                     "d%d: IOMMU mapping dfn %"PRI_dfn" to mfn %"PRI_mfn" failed: %d\n",
> -                   d->domain_id, dfn_x(dfn_add(dfn, i)),
> -                   mfn_x(mfn_add(mfn, i)), rc);
> +                   d->domain_id, dfn_x(dfn), mfn_x(mfn), rc);
> +
> +        for ( j = 0; j < i; j += 1UL << order )
> +        {
> +            dfn = dfn_add(dfn0, j);
> +            order = mapping_order(hd, dfn, _mfn(0), i - j);
>  
> -        while ( i-- )
>              /* if statement to satisfy __must_check */
> -            if ( iommu_call(hd->platform_ops, unmap_page, d, dfn_add(dfn, i),
> -                            0, flush_flags) )
> +            if ( iommu_call(hd->platform_ops, unmap_page, d, dfn, order,
> +                            flush_flags) )
>                  continue;
> +        }

Why you need this unmap loop here, can't you just use iommu_unmap?

Thanks, Roger.