Re: [Xen-devel] [PATCH 1/2] xen/x86: return partial memory map in case of not enough space

[Juergen Gross <jgross@xxxxxxxx>]

On 05/12/16 18:17, Jan Beulich wrote:
>>>> On 05.12.16 at 17:34, <JGross@xxxxxxxx> wrote:
>> For XENMEM_machine_memory_map the hypervisor returns EINVAL if the
>> caller's buffer can't hold all entries.
>>
>> This is a problem as the caller has normally a static buffer defined
>> and when he is doing the call no dynamic memory allocation is
>> possible as nothing is yet known about the system's memory layout.
>>
>> Instead of just fail deliver as many memory map entries as possible
>> and return with E2BIG indicating the result was incomplete. Then the
>> caller will be capable to use at least some memory reported to exist
>> to allocate a larger buffer for the complete memory map.
> 
> This makes no sense, as what we're talking about here is the
> machine memory map, and the calling Dom0 kernel can't allocate
> from that pool directly. Instead it would need its own memory
> map to know where to place such a larger buffer, and this map
> is usually just one or two entries large.

This is true. In practice, however, things are a little bit more
complicated:

Linux being started as dom0 tries to rearrange memory layout to match
the one of the physical machine. It will only add memory to its
allocator which is known to either not need to be moved or which has
already been moved. And this decision is based on the machine memory
map.

I admit it is a Linux kernel private decision how to handle the boot and
adding of memory to its allocator. OTOH the "all or nothing" approach of
the hypervisor regarding delivery of the machine memory map is a little
bit strange, especially as the BIOS is returning the E820 map one entry
after the other.

> For that reason I'm not convinced we need or want this change.

It would certainly make it easier for the Linux kernel.


Juergen

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