Re: [Xen-devel] [PATCH v8 7/7] vtd: add lookup_page method to iommu_ops

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.09.18 at 12:31, <paul.durrant@xxxxxxxxxx> wrote:
> This patch adds a new method to the VT-d IOMMU implementation to find the
> MFN currently mapped by the specified DFN along with a wrapper function
> in generic IOMMU code to call the implementation if it exists.

Would you mind clarifying what the use of this is when ...

> +static int intel_iommu_lookup_page(struct domain *d, dfn_t dfn, mfn_t *mfn,
> +                                   unsigned int *flags)
> +{
> +    struct domain_iommu *hd = dom_iommu(d);
> +    struct dma_pte *page, val;
> +    u64 pg_maddr;
> +
> +    /*
> +     * If VT-d shares EPT page table or if the domain is the hardware
> +     * domain and iommu_passthrough is set then pass back the dfn.
> +     */
> +    if ( iommu_use_hap_pt(d) ||
> +         (iommu_passthrough && is_hardware_domain(d)) )
> +    {
> +        *mfn = _mfn(dfn_x(dfn));
> +        return 0;
> +    }
> +
> +    spin_lock(&hd->arch.mapping_lock);
> +
> +    pg_maddr = addr_to_dma_page_maddr(d, dfn_to_daddr(dfn), 0);
> +    if ( !pg_maddr )
> +    {
> +        spin_unlock(&hd->arch.mapping_lock);
> +        return -ENOMEM;
> +    }
> +
> +    page = map_vtd_domain_page(pg_maddr);
> +    val = page[dfn_x(dfn) & LEVEL_MASK];
> +
> +    unmap_vtd_domain_page(page);
> +    spin_unlock(&hd->arch.mapping_lock);
> +
> +    if ( !dma_pte_present(val) )
> +        return -ENOENT;
> +
> +    *mfn = maddr_to_mfn(dma_pte_addr(val));
> +    *flags = dma_pte_read(val) ? IOMMUF_readable : 0;
> +    *flags |= dma_pte_write(val) ? IOMMUF_writable : 0;
> +
> +    return 0;
> +}

... the locking used here suggests that the result is stale by the
time the caller gets to look at it? If this relies on locking in the
callers, then I think this should be spelled out in comments next
to the function definitions and/or declarations.

Jan



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