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Re: [PATCH 33/36] doc, xen-command-line: introduce coloring options



Hi Marco,

On 04/03/2022 17:46, Marco Solieri wrote:
From: Luca Miccio <lucmiccio@xxxxxxxxx>

Four additional parameters in the Xen command line are used to define
the underlying coloring policy, which is not directly configurable
otherwise.

Signed-off-by: Luca Miccio <lucmiccio@xxxxxxxxx>
Signed-off-by: Marco Solieri <marco.solieri@xxxxxxxxxxxxxxx>
---
  docs/misc/xen-command-line.pandoc | 51 +++++++++++++++++++++++++++++--
  1 file changed, 49 insertions(+), 2 deletions(-)

diff --git a/docs/misc/xen-command-line.pandoc 
b/docs/misc/xen-command-line.pandoc
index efda335652..a472d51cf9 100644
--- a/docs/misc/xen-command-line.pandoc
+++ b/docs/misc/xen-command-line.pandoc
@@ -299,6 +299,20 @@ can be maintained with the pv-shim mechanism.
      cause Xen not to use Indirect Branch Tracking even when support is
      available in hardware.
+### buddy\_size (arm64)
+> `= <size in megabyte>`
+
+> Default: `64 MB`
+
+Amount of memory reserved for the buddy allocator when colored allocator is
+active. This options is useful only if coloring support is enabled.
+The colored allocator is meant as an alternative to the buddy allocator,
+since its allocation policy is by definition incompatible with the
+generic one. Since the Xen heap systems is not colored yet, we need to
+support the coexistence of the two allocators for now. This parameter, which is
+optional and for expert only, is used to set the amount of memory reserved to
+the buddy allocator.

A few questions:
  - How did you chose the default?
  - How can a user decide the size of the buddy_size?

+
  ### clocksource (x86)
  > `= pit | hpet | acpi | tsc`
@@ -884,7 +898,17 @@ Controls for the dom0 IOMMU setup. Incorrect use of this option may result in a malfunctioning system. -### dom0_ioports_disable (x86)

This change sounds unrelated to the patch itself. I would also expect that we would want to backport it. So can this be backported.

+### dom0\_colors (arm64)
+> `= List of <integer>-<integer>`
+
+> Default: `All available colors`
+
+Specify dom0 color configuration. If the parameter is not set, all available
+colors are chosen and the user is warned on Xen's serial console. This color
+configuration acts also as the default one for all DomUs that do not have any
+explicit color assignment in their configuration file.
+
+### dom0\_ioports\_disable (x86)
  > `= List of <hex>-<hex>`
Specify a list of IO ports to be excluded from dom0 access.
@@ -2625,6 +2649,20 @@ unknown NMIs will still be processed.
  Set the NMI watchdog timeout in seconds.  Specifying `0` will turn off
  the watchdog.
+### way\_size (arm64)
+> `= <size in byte>`
+
+> Default: `Obtained from the hardware`
+
+Specify the way size of the Last Level Cache. This parameter is only useful 
with
+coloring support enabled. It is an optional, expert-only parameter and it is
+used to calculate what bits in the physical address can be used by the coloring
+algorithm, and thus the maximum available colors on the platform. It can be
+obtained by dividing the total LLC size by the number of associativity ways.
+By default, the value is also automatically computed during coloring
+initialization to avoid any kind of misconfiguration. For this reason, it is
+highly recommended to use this boot argument with specific needs only.

Given the last two sentences, why would someone wants to use it?

+
  ### x2apic (x86)
  > `= <boolean>`
@@ -2642,7 +2680,16 @@ In the case that x2apic is in use, this option switches between physical and
  clustered mode.  The default, given no hint from the **FADT**, is cluster
  mode.
-### xenheap_megabytes (arm32)

Same here.

+### xen\_colors (arm64)
+> `= List of <integer>-<integer>`
+
+> Default: `0-0: the lowermost color`
+
+Specify Xen color configuration.
+Two colors are most likely needed on platforms where private caches are
+physically indexed, e.g. the L1 instruction cache of the Arm Cortex-A57.

How can someone decide the number of colors to be used for Xen?

Cheers,

--
Julien Grall



 


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