Re: [git pull] IOMMU Fixes for Linux v5.7-rc4

[Linus Torvalds]

On Sun, May 10, 2020 at 5:26 AM Joerg Roedel <joro@xxxxxxxxxx> wrote:
>
>            The first race condition was around
>           the non-atomic update of the domain page-table root pointer
>           and the variable containing the page-table depth (called
>           mode). This is fixed now be merging page-table root and mode
>           into one 64-bit field which is read/written atomically.

This seems a bit odd.

The pointer part is always page-aligned, and the "mode" is just three bits.

Why isn't it just encoded as one pointer with the low three bits being the mode?

The thing is, the 64-bit atomic reads/writes are very expensive on
32-bit x86. If it was just a native pointer, it would be much cheaper
than an "atomic64_t".

                Linus