Re: [PATCH 1/3] amd-vi: use the same IOMMU page table levels for PV and HVM

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 17/11/2023 9:47 am, Roger Pau Monne wrote:
> Using different page table levels for HVM or PV guest is not helpful, and is
> not inline with the IOMMU implementation used by the other architecture vendor
> (VT-d).
>
> Switch to uniformly use DEFAULT_DOMAIN_ADDRESS_WIDTH in order to set the 
> AMD-Vi
> page table levels.
>
> Note using the max RAM address for PV was bogus anyway, as there's no 
> guarantee
> there can't be device MMIO or reserved regions past the maximum RAM region.

Indeed - and the MMIO regions do matter for P2P DMA.

> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Variable-height IOMMU pagetables are not worth the security
vulnerabilities they're made of.  I regret not fighting hard enough to
kill them entirely several years ago...

Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, although...

> ---
>  xen/drivers/passthrough/amd/pci_amd_iommu.c | 20 ++++++++------------
>  1 file changed, 8 insertions(+), 12 deletions(-)
>
> diff --git a/xen/drivers/passthrough/amd/pci_amd_iommu.c 
> b/xen/drivers/passthrough/amd/pci_amd_iommu.c
> index 6bc73dc21052..f9e749d74da2 100644
> --- a/xen/drivers/passthrough/amd/pci_amd_iommu.c
> +++ b/xen/drivers/passthrough/amd/pci_amd_iommu.c
> @@ -359,21 +359,17 @@ int __read_mostly amd_iommu_min_paging_mode = 1;
>  static int cf_check amd_iommu_domain_init(struct domain *d)
>  {
>      struct domain_iommu *hd = dom_iommu(d);
> +    int pgmode = amd_iommu_get_paging_mode(
> +        1UL << (DEFAULT_DOMAIN_ADDRESS_WIDTH - PAGE_SHIFT));

"paging mode" comes from the spec, but it's a very backwards way of
spelling height.

Can we at least start to improve the comprehensibility by renaming this
variable.

> +
> +    if ( pgmode < 0 )
> +        return pgmode;
>  
>      /*
> -     * Choose the number of levels for the IOMMU page tables.
> -     * - PV needs 3 or 4, depending on whether there is RAM (including 
> hotplug
> -     *   RAM) above the 512G boundary.
> -     * - HVM could in principle use 3 or 4 depending on how much guest
> -     *   physical address space we give it, but this isn't known yet so use 4
> -     *   unilaterally.
> -     * - Unity maps may require an even higher number.
> +     * Choose the number of levels for the IOMMU page tables, taking into
> +     * account unity maps.
>       */
> -    hd->arch.amd.paging_mode = max(amd_iommu_get_paging_mode(
> -            is_hvm_domain(d)
> -            ? 1UL << (DEFAULT_DOMAIN_ADDRESS_WIDTH - PAGE_SHIFT)
> -            : get_upper_mfn_bound() + 1),
> -        amd_iommu_min_paging_mode);
> +    hd->arch.amd.paging_mode = max(pgmode, amd_iommu_min_paging_mode);

I think these min/max variables can be dropped now we're not doing
variable height IOMMU pagetables, which further simplifies this expression.

Dunno if it's something better folded into this patch, or done at some
point in the future.

~Andrew