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docs: Update documentation for hugepage_size option

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Including a warning that the user is respsonsible for ensuring that they
have sufficient pages of the specified size.

Signed-off-by: Rob Bradford <robert.bradford@intel.com>
This commit is contained in:
Rob Bradford 2021-02-04 16:22:55 +00:00
parent 29607f38ad
commit b5fb744828
1 changed files with 7 additions and 5 deletions
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12
docs/memory.md
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@ -15,6 +15,7 @@ struct MemoryConfig {
mergeable: bool,
shared: bool,
hugepages: bool,
hugepage_size: Option<u64>,
hotplug_method: HotplugMethod,
hotplug_size: Option<u64>,
hotplugged_size: Option<u64>,
@ -76,23 +77,24 @@ _Example_
--memory size=1G,shared=on
```
### `hugepages`
### `hugepages` and `hugepage_size`
Specifies if the memory must be `mmap(2)` with `MAP_HUGETLB` and `MAP_HUGE_2MB`
flags. This performs a memory mapping relying on 2MiB pages instead of the
default 4kiB pages.
Specifies if the memory must be created and `mmap(2)` with `MAP_HUGETLB` and size
flags. This performs a memory mapping relying on the specified huge page size. If no huge page size is supplied the system's default huge page size is used.
By using hugepages, one can improve the overall performance of the VM, assuming
the guest will allocate hugepages as well. Another interesting use case is VFIO
as it speeds up the VM's boot time since the amount of IOMMU mappings are
reduced.
The user is responsible for ensuring there are sufficient huge pages of the specified size for the VMM to use. Failure to do so may result in strange VMM behaviour.
By default this option is turned off.
_Example_
```
--memory size=1G,hugepages=on
--memory size=1G,hugepages=on,hugepage_size=2M
```
### `hotplug_method`