Re: [PATCH] x86/pv: limit GDT and LDT mappings areas to max number of vCPUs

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Nov 21, 2024 at 12:12:18PM +0100, Roger Pau Monne wrote:
> The allocation of the paging structures in the per-domain area for mapping the
> guest GDT and LDT can be limited to the maximum number of vCPUs the guest can
> have.  The maximum number of vCPUs is available at domain creation since 
> commit
> 4737fa52ce86.
> 
> Limiting to the actual number of vCPUs avoids wasting memory for paging
> structures that will never be used.  Current logic unconditionally uses 513
> pages, one page for the L3, plus 512 L1 pages.

This is not true, I was confused with the logic in
create_perdomain_mapping().  When create_perdomain_mapping() is called
with pl1tab == NULL and ppg == NULL it just allocates the L2, but not
the L1 tables.

So the purpose of the create_perdomain_mapping(d, GDT_LDT_VIRT_START,
...) in pv_domain_initialise() is even more dubious now - as it just
allocates a page to use as L2.  I'm tempted to just remove it if you
agree, since I don't consider this useful.  The allocation will
already be done at vCPU initialization.

Thanks, Roger.