Re: [PATCH for-4.17 1/2] docs: Document the minimal requirement of static heap

[Julien Grall <julien@xxxxxxx>]

Hi Henry,

On 13/10/2022 03:57, Henry Wang wrote:
> The static heap feature requires user to know the minimal size of
> heap to make sure the system can work. Since the heap controlled
> by Xen is intended to provide memory for the whole system, not only
> the boot time memory allocation should be covered by the static
> heap region, but also the runtime allocation should be covered.
>
> The main source of runtime allocation is the memory for the P2M.
> Currently, from XSA-409, the P2M memory is bounded by the P2M pool.
> So make this part as the minimal requirement of static heap. The
> amount of memory allocated after all the guests have been created
> should be quite limited and mostly predictable.
>
> This commit adds documentation that explains how a user can size the
> static heap region.
>
> Signed-off-by: Henry Wang <Henry.Wang@xxxxxxx>
> ---
>   docs/misc/arm/device-tree/booting.txt | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> diff --git a/docs/misc/arm/device-tree/booting.txt 
> b/docs/misc/arm/device-tree/booting.txt
> index 87eaa3e254..046f28ce31 100644
> --- a/docs/misc/arm/device-tree/booting.txt
> +++ b/docs/misc/arm/device-tree/booting.txt
> @@ -531,6 +531,13 @@ Below is an example on how to specify the static heap in 
> device tree:
>   RAM starting from the host physical address 0x30000000 of 1GB size will
>   be reserved as static heap.
>   
> +Users should be mindful that the static heap should at least satisfy the
> +allocation of the P2M maps for all guests. Currently, the minimal requirement
> +of per-domain P2M pages pool is in-sync with function
> +libxl__get_required_paging_memory() (for xl-created domUs) and
> +domain_p2m_pages() (for dom0less domUs), that is, 1MB per vCPU, plus 4KiB per
> +MiB of RAM for the P2M map, and plus 512KiB to cover extended regions.
I think this wording is OK if the feature is a tech preview. However, if 
this is security supported, we need to provide some more details about 
the size.
In particular, this doesn't tell a user how they can find the size that 
would fit them. Can this be decided with a formula?
If not, could a user configure his system without the reserved heap and 
find out the memory usage from dom0 (or a debug keys)?
Cheers,


--
Julien Grall