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 11:16 AM, George Dunlap <george.dunlap@xxxxxxxxxx> wrote:
> 
> 
> 
>> 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.

Er, just in case it wasn’t clear — I agree with this:

>> I continue to think that the decision should only be based on physical vs 
>> virtually contiguous.


 -George