Re: [PATCH 04/22] x86/mm: ensure L4 idle_pg_table is not modified past boot

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

On Tue, Sep 10, 2024 at 11:00:27AM +0200, Jan Beulich wrote:
> On 10.09.2024 10:54, Roger Pau Monné wrote:
> > On Tue, Aug 13, 2024 at 05:54:54PM +0200, Jan Beulich wrote:
> >> On 26.07.2024 17:21, Roger Pau Monne wrote:
> >>> The idle_pg_table L4 is cloned to create all the other L4 Xen uses, and 
> >>> hence
> >>> it shouldn't be modified once further L4 are created.
> >>
> >> Yes, but the window between moving into SYS_STATE_smp_boot and Dom0 having
> >> its initial page tables created is quite large. If the justification was
> >> relative to AP bringup, that may be all fine. But when related to cloning,
> >> I think that would then truly want keying to there being any non-system
> >> domain(s).
> > 
> > Further changes in this series will add a per-CPU idle page table, and
> > hence we need to ensure that by the time APs are started the BSP L4 idle
> > page directory is not changed, as otherwise the copies in the APs
> > would get out of sync.
> > 
> > The idle system domain is indeed tied to the idle page talbes, but the
> > idle vCPU0 (the BSP) directly uses idle_pg_table (no copying), and
> > hence it's fine to allow modifications of the L4 idle page table
> > directory up to when APs are started (those will indeed make copies of
> > the idle L4.
> 
> Which may want at least mentioning in the description then. I take it
> that ...
> 
> >>> --- a/xen/arch/x86/mm.c
> >>> +++ b/xen/arch/x86/mm.c
> >>> @@ -5023,6 +5023,12 @@ static l3_pgentry_t *virt_to_xen_l3e(unsigned long 
> >>> v)
> >>>          mfn_t l3mfn;
> >>>          l3_pgentry_t *l3t = alloc_mapped_pagetable(&l3mfn);
> >>>  
> >>> +        /*
> >>> +         * dom0 is build at smp_boot, at which point we already create 
> >>> new L4s
> >>> +         * based on idle_pg_table.
> >>> +         */
> 
> ... this comment is then refined by the later patches you refer to?

Hm, I would have to double check, not sure I've updated it once the
idle_pg_table is cloned for AP bringup.

Will expand commit message and update the comment here if not done
already by later patches.

Thanks, Roger.