RE: [PATCH 6/6] VT-d: avoid allocating domid_{bit,}map[] when possible

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich <jbeulich@xxxxxxxx>
> Sent: Friday, November 12, 2021 5:50 PM
> 
> When an IOMMU implements the full 16 bits worth of DID in context
> entries, there's no point going through a memory base translation table.
> For IOMMUs not using Caching Mode we can simply use the domain IDs
> verbatim, while for Caching Mode we need to avoid DID 0.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> ---
> For the case where the memory tables are needed, xvzalloc_array() would
> of course be an option to use here as well, despite this being boot time
> allocations. Yet the introduction of xvmalloc() et al continues to be
> stuck ...
> 
> --- a/xen/drivers/passthrough/vtd/iommu.c
> +++ b/xen/drivers/passthrough/vtd/iommu.c
> @@ -62,11 +62,32 @@ static struct tasklet vtd_fault_tasklet;
>  static int setup_hwdom_device(u8 devfn, struct pci_dev *);
>  static void setup_hwdom_rmrr(struct domain *d);
> 
> +static bool domid_mapping(const struct vtd_iommu *iommu)
> +{
> +    return (const void *)iommu->domid_bitmap != (const void *)iommu-
> >domid_map;
> +}
> +
> +static domid_t convert_domid(const struct vtd_iommu *iommu, domid_t
> domid)
> +{
> +    /*
> +     * While we need to avoid DID 0 for caching-mode IOMMUs, maintain
> +     * the property of the transformation being the same in either
> +     * direction. By clipping to 16 bits we ensure that the resulting
> +     * DID will fit in the respective context entry field.
> +     */
> +    BUILD_BUG_ON(sizeof(domid_t) > sizeof(uint16_t));
> +
> +    return !cap_caching_mode(iommu->cap) ? domid : ~domid;

If DOMID_MASK grows to 0xFFFF (though unlikely), then it translates 
to '0' when caching mode is true. We need extend BUILD_BUG_ON() to 
check DOMID_MASK in this case, since caching mode implies
total_size minus one for available domain IDs 

Thanks
Kevin