Re: [PATCH DO NOT APPLY] docs: Document allocator properties and the rubric for using them

[George Dunlap <George.Dunlap@xxxxxxxxxx>]

> On Feb 16, 2021, at 10:55 AM, Julien Grall <julien@xxxxxxx> wrote:
> 
> Hi George,
> 
> On 16/02/2021 10:28, George Dunlap wrote:
>> Document the properties of the various allocators and lay out a clear
>> rubric for when to use each.
>> Signed-off-by: George Dunlap <george.dunlap@xxxxxxxxxx>
>> ---
>> This doc is my understanding of the properties of the current
>> allocators (alloc_xenheap_pages, xmalloc, and vmalloc), and of Jan's
>> proposed new wrapper, xvmalloc.
>> xmalloc, vmalloc, and xvmalloc were designed more or less to mirror
>> similar functions in Linux (kmalloc, vmalloc, and kvmalloc
>> respectively).
>> CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> CC: Jan Beulich <jbeulich@xxxxxxxx>
>> CC: Roger Pau Monne <roger.pau@xxxxxxxxxx>
>> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
>> CC: Julien Grall <julien@xxxxxxx>
>> ---
>>  .../memory-allocation-functions.rst           | 118 ++++++++++++++++++
>>  1 file changed, 118 insertions(+)
>>  create mode 100644 docs/hypervisor-guide/memory-allocation-functions.rst
>> diff --git a/docs/hypervisor-guide/memory-allocation-functions.rst 
>> b/docs/hypervisor-guide/memory-allocation-functions.rst
>> new file mode 100644
>> index 0000000000..15aa2a1a65
>> --- /dev/null
>> +++ b/docs/hypervisor-guide/memory-allocation-functions.rst
>> @@ -0,0 +1,118 @@
>> +.. SPDX-License-Identifier: CC-BY-4.0
>> +
>> +Xenheap memory allocation functions
>> +===================================
>> +
>> +In general Xen contains two pools (or "heaps") of memory: the *xen
>> +heap* and the *dom heap*.  Please see the comment at the top of
>> +``xen/common/page_alloc.c`` for the canonical explanation.
>> +
>> +This document describes the various functions available to allocate
>> +memory from the xen heap: their properties and rules for when they should be
>> +used.
>> +
>> +
>> +TLDR guidelines
>> +---------------
>> +
>> +* By default, ``xvmalloc`` (or its helper cognates) should be used
>> +  unless you know you have specific properties that need to be met.
>> +
>> +* If you need memory which needs to be physically contiguous, and may
>> +  be larger than ``PAGE_SIZE``...
>> +
>> +  - ...and is order 2, use ``alloc_xenheap_pages``.
>> +
>> +  - ...and is not order 2, use ``xmalloc`` (or its helper cognates)..
>> +
>> +* If you don't need memory to be physically contiguous, and know the
>> +  allocation will always be larger than ``PAGE_SIZE``, you may use
>> +  ``vmalloc`` (or one of its helper cognates).
>> +
>> +* If you know that allocation will always be less than ``PAGE_SIZE``,
>> +  you may use ``xmalloc``.
> 
> AFAICT, the determining factor is PAGE_SIZE. This is a single is a single 
> value on x86 (e.g. 4KB) but on other architecture this may be multiple values.
> 
> For instance, on Arm, this could be 4KB, 16KB, 64KB (note that only the 
> former is so far supported on Xen).
> 
> For Arm and common code, it feels to me we can't make a clear decision based 
> on PAGE_SIZE. Instead, I continue to think that the decision should only be 
> based on physical vs virtually contiguous.
> 
> We can then add further rules for x86 specific code if the maintainers want.

Sorry my second mail was somewhat delayed — my intent was: 1) post the document 
I’d agreed to write, 2) say why I think the proposal is a bad idea. :-)

Re page size — the vast majority of time we’re talking “knowing” that the size 
is less than 4k.  If we’re confident that no architecture will ever have a page 
size less than 4k, then we know that all allocations less than 4k will always 
be less than PAGE_SIZE.  Obviously larger page sizes then becomes an issue.

But in any case — unless we have BUG_ON(size > PAGE_SIZE), we’re going to have 
to have a fallback, which is going to cost one precious conditional, making the 
whole exercise pointless.

 -George