Re: [kvm-unit-tests PATCH 1/8] x86: Avoid top-most page for vmalloc on x86-64

[Chao Gao <chao.gao@intel.com>]

On Fri, Jun 20, 2025 at 05:39:05PM +0200, Mathias Krause wrote:
>The x86-64 implementation of setup_mmu() doesn't initialize 'vfree_top'
>and leaves it at its zero-value. This isn't wrong per se, however, it
>leads to odd configurations when the first vmalloc/vmap page gets
>allocated. It'll be the very last page in the virtual address space --
>which is an interesting corner case -- but its boundary will probably
>wrap. It does so for CET's shadow stack, at least, which loads the
>shadow stack pointer with the base address of the mapped page plus its
>size, i.e. 0xffffffff_fffff000 + 4096, which wraps to 0x0.
>
>The CPU seems to handle such configurations just fine. However, it feels
>odd to set the shadow stack pointer to "NULL".

Not sure if adjusting this is necessary. As a unit test, exercising this corner
case might be beneficial. But I don't have a strong opinion. So,

Reviewed-by: Chao Gao <chao.gao@intel.com>

>
>To avoid the wrapping, ignore the top most page by initializing
>'vfree_top' to just one page below.

Nit: this makes the comment in test_lam_sup() stale, specifically "KUT
initializes vfree_top to 0 for X86_64". So, that comment needs an update.