Re: [PATCH v7 13/20] x86/virt/tdx: Allocate and set up PAMTs for TDMRs

[Huang, Kai]

On Wed, 2022-11-23 at 14:57 -0800, Dave Hansen wrote:
> On 11/20/22 16:26, Kai Huang wrote:
> > The TDX module uses additional metadata to record things like which
> > guest "owns" a given page of memory.  This metadata, referred as
> > Physical Address Metadata Table (PAMT), essentially serves as the
> > 'struct page' for the TDX module.  PAMTs are not reserved by hardware
> > up front.  They must be allocated by the kernel and then given to the
> > TDX module.
> 
> ... during module initialization.

Thanks.

> 
> > TDX supports 3 page sizes: 4K, 2M, and 1G.  Each "TD Memory Region"
> > (TDMR) has 3 PAMTs to track the 3 supported page sizes.  Each PAMT must
> > be a physically contiguous area from a Convertible Memory Region (CMR).
> > However, the PAMTs which track pages in one TDMR do not need to reside
> > within that TDMR but can be anywhere in CMRs.  If one PAMT overlaps with
> > any TDMR, the overlapping part must be reported as a reserved area in
> > that particular TDMR.
> > 
> > Use alloc_contig_pages() since PAMT must be a physically contiguous area
> > and it may be potentially large (~1/256th of the size of the given TDMR).
> > The downside is alloc_contig_pages() may fail at runtime.  One (bad)
> > mitigation is to launch a TD guest early during system boot to get those
> > PAMTs allocated at early time, but the only way to fix is to add a boot
> > option to allocate or reserve PAMTs during kernel boot.
> 
> FWIW, we all agree that this is a bad permanent way to leave things.
> You can call me out here as proposing that this wart be left in place
> while this series is merged and is a detail we can work on afterword
> with new module params, boot options, Kconfig or whatever.

Sorry do you mean to call out in the cover letter, or in this changelog?

> > TDX only supports a limited number of reserved areas per TDMR to cover
> > both PAMTs and memory holes within the given TDMR.  If many PAMTs are
> > allocated within a single TDMR, the reserved areas may not be sufficient
> > to cover all of them.
> > 
> > Adopt the following policies when allocating PAMTs for a given TDMR:
> > 
> >   - Allocate three PAMTs of the TDMR in one contiguous chunk to minimize
> >     the total number of reserved areas consumed for PAMTs.
> >   - Try to first allocate PAMT from the local node of the TDMR for better
> >     NUMA locality.
> > 
> > Also dump out how many pages are allocated for PAMTs when the TDX module
> > is initialized successfully.
> 
> ... this helps answer the eternal "where did all my memory go?" questions.

Will add to the comment.

[...]


> > +/*
> > + * Pick a NUMA node on which to allocate this TDMR's metadata.
> > + *
> > + * This is imprecise since TDMRs are 1G aligned and NUMA nodes might
> > + * not be.  If the TDMR covers more than one node, just use the _first_
> > + * one.  This can lead to small areas of off-node metadata for some
> > + * memory.
> > + */
> > +static int tdmr_get_nid(struct tdmr_info *tdmr)
> > +{
> > +	struct tdx_memblock *tmb;
> > +
> > +	/* Find the first memory region covered by the TDMR */
> > +	list_for_each_entry(tmb, &tdx_memlist, list) {
> > +		if (tmb->end_pfn > (tdmr_start(tdmr) >> PAGE_SHIFT))
> > +			return tmb->nid;
> > +	}
> 
> Aha, the first use of tmb->nid!  I wondered why that was there.

As you suggested I'll introduce the nid member of 'tdx_memblock' in this patch. 

> 
> > +
> > +	/*
> > +	 * Fall back to allocating the TDMR's metadata from node 0 when
> > +	 * no TDX memory block can be found.  This should never happen
> > +	 * since TDMRs originate from TDX memory blocks.
> > +	 */
> > +	WARN_ON_ONCE(1);
> 
> That's probably better a pr_warn() or something.  A backtrace and all
> that jazz seems a bit overly dramatic for this.

How about below?

pr_warn("TDMR [0x%llx, 0x%llx): unable to find local NUMA node for PAMT
allocation, fallback to use node 0.\n");