Re: [PATCH v2 1/3] KVM: arm64: Compute hyp early-allocation and pool sizes in 64 bits

[Mostafa Saleh]

On Fri, Nov 06, 2026 at 11:31:57AM +0000, Fuad Tabba wrote:
> Hi Xuheng,
> 
> On Thu, 05 Nov 2026 22:59:48 +0000, Xuheng Li <xuheng@xxxxxxxxxxxxxxx> wrote:
> > hyp_early_alloc_contig() and hyp_pool_init() both take a page count as
> > an unsigned int and compute the size in bytes as nr_pages << PAGE_SHIFT
> > in 32 bits.  For 4 GiB or more (nr_pages >= 2^20 with 4 KiB pages) the
> > shift overflows.
> 
> Mostafa (Cc'd) posted a fix for the same truncation in July [1],
> widening the parameters to unsigned long instead. Would you be OK
> picking up his patch for v3, keeping him as the author and your
> description and tags?
> 

I was preparing v3 anyway, I can post it today.

Thanks,
Mostafa

> > Two of the regions that pKVM carves out at boot are sized from the
> > amount of memory: hyp_s1_pgtable_pages() for the hyp page tables (hpool)
> > and host_s2_pgtable_pages() for the host stage-2 pool both provision
> > page-granular tables for all of memory, about one page per 2 MiB of
> > RAM, so they reach 4 GiB on a machine with about 2 TiB of RAM.  Arm
> > systems of that size exist, for example AWS Graviton4 x8g.metal-48xl
> > instances (3 TiB) and Ampere Altra Max servers (up to 4 TiB per
> > socket).
> 
> Could you also list the hyp vmemmap here? It's sized the same way and
> overflows at 2 TiB too.
> 
> > On such a machine, divide_memory_pool() gets a truncated hyp page-table
> > region from hyp_early_alloc_contig(), so the host stage-2 region and the
> > FF-A proxy pages that follow it start inside the range that hpool will
> > own.  hyp_pool_init() then attaches all nr_pages pages of each pool to
> > its free lists, so hpool and host_s2_pool share pages and the same page
> > can be handed out twice, for example as a hyp stage-1 table and as a
> > host stage-2 table.  hyp_pool_init() also derives range_end and
> > max_order from the truncated size, so most of each pool is treated as
> > external order-0 pages; on a host whose IPA size makes the host stage-2
> > PGD a multi-page allocation (for example a 42-bit PARange), that
> > allocation fails and host_s2_zalloc_pages_exact() uses the NULL result.
> 
> Could you describe here what actually happens? I think pKVM init fails
> before any page is handed out twice. Just below 2 TiB, hyp_pool_init()
> for host_s2_pool hits the BUG_ON() in hyp_set_page_refcounted() on
> hpool's page-table pages, and from 2 TiB up hyp_back_vmemmap() zeroes
> the new hyp page tables while they're being built.
> 
> Cheers,
> /fuad
> 
> [1] https://lore.kernel.org/all/20260715150125.2798116-1-smostafa@xxxxxxxxxx/