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Re: [PATCH] x86/boot: Load microcode much earlier on boot


  • To: Jan Beulich <jbeulich@xxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Thu, 21 Nov 2024 13:52:25 +0000
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  • Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, "Daniel P . Smith" <dpsmith@xxxxxxxxxxxxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Thu, 21 Nov 2024 13:52:37 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 20/11/2024 10:44 am, Jan Beulich wrote:
> On 19.11.2024 22:57, Andrew Cooper wrote:
>> Resolve the todo, and move microcode loading to be the eariest action after
>> establishing a console.
> So yes, this is merely strengthening a dependency we already have:
> bootstrap_map_addr() arranging to avoid allocating any page table memory.
> Yet don't we rather need to move away from this? We don't know what's in
> the excess space we map, and hence we better wouldn't have any (cachable)
> mappings thereof.

I don't see it this way.

Yes it is somewhat dodgy that bootstrap_map*() blindly uses 2M pages.

It's a problem with cacheability (in theory at least; TLBs tend to
splinter mappings on MTRR mismatch).  It's a hard problem with the RMP
table (AMD SEV-SNP), where any access into the page yields #PF[Rsvd] if
there's any overlap with the RMP (locked in place by firmware and not
always 2M aligned).

But, any fix to this needs to fix *all* users of bootstrap_map*(),
including the move_memory() loop.

So I don't see any interaction with how early or late the early-map
infrastructure is used.

~Andrew

P.S. I have a few ideas on how to address this.

With only 2 extra pagetables in .init.bss, we can support any arbitrary
single mapping with 4k alignment.

move_memory() is the only user that makes multiple mappings, and it's
broken anyway.  I'm now certain that it does get memmove() wrong if the
move has a partial overlap.

However, move_memory() always has a good destination pointer in the
directmap, so I've got a crazy idea to fix the memmove() problem by also
hooking the bootmap into idle_pg_table[511] creating a high alias of the
mapping, and the source pointer can choose whichever of the aliases are
the correct side of the directmap.



 


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