Re: [PATCH] arm64/boot: Zero-initialize idmap PGDs before use

[Marc Zyngier <maz@kernel.org>]

Hi Sam,

On Sun, 24 Aug 2025 04:05:05 +0100,
Sam Edwards <cfsworks@gmail.com> wrote:
> 
> On Sat, Aug 23, 2025 at 5:29 PM Ard Biesheuvel <ardb@kernel.org> wrote:
> >

[...]

> > Under which conditions would PGD_SIZE assume a value greater than PAGE_SIZE?
> 
> I might be doing my math wrong, but wouldn't 52-bit VA with 4K
> granules and 5 levels result in this?

No. 52bit VA at 4kB granule results in levels 0-3 each resolving 9
bits, and level -1 resolving 4 bits. That's a total of 40 bits, plus
the 12 bits coming directly from the VA making for the expected 52.

> Each PTE represents 4K of virtual memory, so covers VA bits [11:0]
> (this is level 3)

That's where you got it wrong. The architecture is pretty clear that
each level resolves PAGE_SHIFT-3 bits, hence the computation
above. The bottom PAGE_SHIFT bits are directly extracted from the VA,
without any translation.

> Each PMD has 512 PTEs, the index of which covers VA bits [20:12] (this
> is level 2)
> Each PUD references 512 PMDs, the index covering VA [29:21] (this is level 1)
> Each P4D references 512 PUDs, indexed by VA [38:30] (this is level 0)
> The PGD, at level -1, therefore has to cover VA bits [51:39], which
> means it has a 13-bit index: 8192 entries of 8 bytes each would make
> it 16 pages in size.
>
> > Note that at stage 1, arm64 does not support page table concatenation,
> > and so the root page table is never larger than a page.
> 
> Doesn't PGD_SIZE refer to the size used for userspace PGDs after the
> boot progresses beyond stage 1? (What do you mean by "never" here?
> "Under no circumstances is it larger than a page at stage 1"? Or
> "during the entire lifecycle of the system, there is no time at which
> it's larger than a page"?)

Never, ever, is a S1 table bigger than a page. This concept doesn't
exist in the architecture. Only S2 tables can use concatenation at the
top-most level, for up to 16 pages (in order to skip a level when
possible).

The top-level can be smaller than a page, with some alignment
constraints, but that's about the only degree of freedom you have for
S1 page tables.

Thanks,

	M.

-- 
Jazz isn't dead. It just smells funny.