Re: [PATCH v3 7/8] x86/virt/tdx: Reduce TDMR's reserved areas by using CMRs to find memory holes

["Huang, Kai" <kai.huang@intel.com>]

On Fri, 2024-08-30 at 13:50 +0300, Hunter, Adrian wrote:
> On 27/08/24 10:14, Kai Huang wrote:
> > A TDX module initialization failure was reported on a Emerald Rapids
> > platform:
> > 
> >   virt/tdx: initialization failed: TDMR [0x0, 0x80000000): reserved areas exhausted.
> >   virt/tdx: module initialization failed (-28)
> > 
> > As part of initializing the TDX module, the kernel informs the TDX
> > module of all "TDX-usable memory regions" using an array of TDX defined
> > structure "TD Memory Region" (TDMR).  Each TDMR must be in 1GB aligned
> > and in 1GB granularity, and all "non-TDX-usable memory holes" within a
> > given TDMR must be marked as "reserved areas".  The TDX module reports a
> > maximum number of reserved areas that can be supported per TDMR.
> 
> The statement:
> 
> 	... all "non-TDX-usable memory holes" within a
> 	given TDMR must be marked as "reserved areas".
> 
> is not exactly true, which is essentially the basis of this fix.

Hmm I think I see what you mean.  Perhaps the "must be marked as" confuses
you?

The "TDX-usable memory" here means all pages that can potentially be used by
TDX.  They don't have to be actually used by TDX, i.e., "TDX-usable memory" vs
"TDX-used memory".

And the "non-TDX-usable memory holes" means the memory regions that cannot be
possibly used by TDX at all.

Is below better if I change "must be marked as" to "are"?

  As part of initializing the TDX module, the kernel informs the TDX
  module of all "TDX-usable memory regions" using an array of TDX defined
  structure "TD Memory Region" (TDMR).  Each TDMR must be in 1GB aligned
  and in 1GB granularity, and all "non-TDX-usable memory holes" within a
  given TDMR are marked as "reserved areas".  The TDX module reports a 
  maximum number of reserved areas that can be supported per TDMR.

Note in my logic here we don't need to mention CMR.  Here I just want to tell
the fact that each TDMR has number of "reserved areas" and the maximum number
is reported by TDX module.

> 
> The relevant requirements are (from the spec):
> 
>   Any non-reserved 4KB page within a TDMR must be convertible
>   i.e., it must be within a CMR

Yes.

> 
>   Reserved areas within a TDMR need not be within a CMR.

Yes.  They need not to be, but they can be.

> 
>   PAMT areas must not overlap with TDMR non-reserved areas;
>   however, they may reside within TDMR reserved areas
>   (as long as these are convertible).

Yes.  However in implementation PAMTs are out of page allocator so they are
all within TDMRs thus need to be put to reserved areas.

Those are TDX architectural requirements.  They are not all related to the fix
of this problem.  The most important thing here is:

  Any non-reserved memory within a TDMR must be within CMR.

That means as long as one memory region is CMR, it doesn't need to be in
"reserved area" from TDX architecture's prespective.  

That means we can include more memory regions (even they cannot be used by TDX
at all) as "non-reserved" areas in TDMRs to reduce the number of "reserved
areas" as long as those regions are within CMR.

This is the logic behind this fix.
 
> 
> > 
> > Currently, the kernel finds those "non-TDX-usable memory holes" within a
> > given TDMR by walking over a list of "TDX-usable memory regions", which
> > essentially reflects the "usable" regions in the e820 table (w/o memory
> > hotplug operations precisely, but this is not relevant here).
> 
> But including e820 table regions that are not "usable" in the TDMR
> reserved areas is not necessary - it is not one of the rules.

True.  That's why we can do this fix.

> 
> What confused me initially was that I did not realize the we already
> require that the TDX Module does not touch memory in the TDMR
> non-reserved areas not specifically allocated to it.  So it makes no
> difference to the TDX Module what the pages that have not been allocated
> to it, are used for.
> 
> > 
> > As shown above, the root cause of this failure is when the kernel tries
> > to construct a TDMR to cover address range [0x0, 0x80000000), there
> > are too many memory holes within that range and the number of memory
> > holes exceeds the maximum number of reserved areas.
> > 
> > The E820 table of that platform (see [1] below) reflects this: the
> > number of memory holes among e820 "usable" entries exceeds 16, which is
> > the maximum number of reserved areas TDX module supports in practice.
> > 
> > === Fix ===
> > 
> > There are two options to fix this: 1) reduce the number of memory holes
> > when constructing a TDMR to save "reserved areas"; 2) reduce the TDMR's
> > size to cover fewer memory regions, thus fewer memory holes.
> 
> Probably better to try and get rid of this "two options" stuff and focus
> on how this is a simple and effective fix.

As I mentioned in another reply I would prefer to keep those options since I
believe they can provide a full view to the reviewers.