Re: [Xen-devel] [PATCH 1/2] xen/dom0: Improve documentation for dom0= and dom0-iommu=

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/12/2018 12:08, Jan Beulich wrote:

On 21.12.18 at 00:40, <andrew.cooper3@xxxxxxxxxx> wrote:

--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -636,55 +636,76 @@ trace feature is only enabled in debugging builds of Xen.
 
 Specify the bit width of the DMA heap.
 
-### dom0 (x86)
-> `= List of [ pvh | shadow ]`
+### dom0
+> `= List of [ pvh=<bool>, shadow=<bool> ]`
 
-> Sub-options:
-
-> `pvh`
+> Applicability: x86

Why the new tag, when everything else uses (x86) next to the
option name?

See the commit message of c/s a3a99df44e5405d2092ec59087681765fa4cdee7

The problem is with the generated HTML anchors when trying to cross reference the options.

 ### dom0-iommu
-> `= List of [ passthrough | strict | map-inclusive ]`
-
-This list of booleans controls the iommu usage by Dom0:
-
-* `passthrough`: disables DMA remapping for Dom0. Default is `false`. Note that
-  this option is hard coded to `false` for a PVH Dom0 and any attempt to
-  overwrite it from the command line is ignored.
-
-* `strict`: sets up DMA remapping only for the RAM Dom0 actually got assigned.
-  Default is `false` which means Dom0 will get mappings for all the host
-  RAM except regions in use by Xen. Note that this option is hard coded to
-  `true` for a PVH Dom0 and any attempt to overwrite it from the command line
-  is ignored.
-
-* `map-inclusive`: sets up DMA remapping for all the non-RAM regions below 4GB
-  except for unusable ranges. Use this to work around firmware issues providing
-  incorrect RMRR/IVMD entries. Rather than only mapping RAM pages for IOMMU
-  accesses for Dom0, with this option all pages up to 4GB, not marked as
-  unusable in the E820 table, will get a mapping established. Note that this
-  option is only applicable to a PV Dom0 and is enabled by default on Intel
-  hardware.
-
-* `map-reserved`: sets up DMA remapping for all the reserved regions in the
-  memory map for Dom0. Use this to work around firmware issues providing
-  incorrect RMRR/IVMD entries. Rather than only mapping RAM pages for IOMMU
-  accesses for Dom0, all memory regions marked as reserved in the memory map
-  that don't overlap with any MMIO region from emulated devices will be
-  identity mapped. This option maps a subset of the memory that would be
-  mapped when using the `map-inclusive` option. This option is available to all
-  Dom0 modes and is enabled by default on Intel hardware.
+> `= List of [ passthrough=<bool>, strict=<bool>, map-inclusive=<bool>,
+>              map-reserved=<bool> ]`
+
+Controls for the dom0 IOMMU setup.
+
+*   The `passthrough` boolean is applicable to x86 PV dom0's only and defaults
+    to false.  It controls whether the IOMMU is fully disabled for devices
+    belonging to dom0 (`passthrough=1`), or whether the IOMMU is set up with
+    an identity transform for dom0 (`passthrough=0`) to prevent dom0 from
+    DMA'ing outside of its permitted areas.
+
+    This option is hardwired to false for x86 PVH dom0's (where a non-identity
+    transform is required for dom0 to function), and is ignored for ARM.
+
+*   The `strict` boolean is applicable to x86 PV dom0's only and defaults to
+    false.  It controls whether dom0 can have IOMMU mappings for all domain
+    RAM in the system, or only for its allocated RAM (and grant mappings etc.)
+
+    This option is hardwired to true for x86 PVH dom0's (as RAM belonging to
+    other domains in the system don't live in a compatible address space), and
+    is ignored for ARM.
+
+*   The `map-inclusive` boolean is applicable to x86 PV dom0's, and sets up DMA
+    remapping for all non-RAM regions below 4GB except for unusable ranges.

I don't thinks this is PV-specific, just its default is.

From arch_iommu_hwdom_init():

    /* Inclusive mappings are enabled by default for PV. */
    if ( iommu_hwdom_inclusive == -1 )
        iommu_hwdom_inclusive = is_pv_domain(d);
    /* Reserved IOMMU mappings are enabled by default. */
    if ( iommu_hwdom_reserved == -1 )
        iommu_hwdom_reserved = 1;

    if ( iommu_hwdom_inclusive && !is_pv_domain(d) )
    {
        printk(XENLOG_WARNING
               "IOMMU inclusive mappings are only supported on PV Dom0\n");
        iommu_hwdom_inclusive = 0;
    }

Attempting to use this option with a PVH dom0 will cause Xen to force it off.

~Andrew

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