Re: [RFC PATCH] x86/boot: use zstd -19 if building for 32-bit

[Mingcong Bai]

Hi Ard,

在 2026/7/28 23:58, Ard Biesheuvel 写道:
> On Tue, 28 Jul 2026, at 17:31, Mingcong Bai wrote:
> > When using native build environments (that is, 32-bit x86 chroot/container
> > as build environment for 32-bit x86 binaries), `zstd -22' requests more
> > virtual memory than that could be allowed on 32-bit hosts.
> >
> > Introduce `cmd_zstd_with_size' for CONFIG_X86_32 builds.
> >
> > This is quite possibly just stupid... But there doesn't seem to be a way
> > to detect the host environment. So I'm sending this patch as a means to
> > ask for your opinions.
> >
> > Any help would be greated appreciated.
> >
> > Signed-off-by: Mingcong Bai <jeffbai@xxxxxxx>
> > ---
> >   arch/x86/boot/compressed/Makefile | 5 +++++
> >   scripts/Makefile.lib              | 3 +++
> >   2 files changed, 8 insertions(+)
>
> Doesn't this mean ZSTD -22 is generally broken on 32-bit? How is this
> specific to the kernel?
It's not (and remains unresolved)[1] - as I mentioned in the email, I 
sent this patch out as a means to spark some discussion on how to 
approach this problem.

Nick Terrel mentioned in commit 48f7ddf785af ("init: Add support for 
zstd compressed kernel") that "zstd22 is used for kernel compression 
because it is decompressed in a single pass, so zstd doesn't need to 
allocate a window buffer." But I wonder if it resulted in tangible 
change in compression ratio - but I can do more testing.

[1]: https://github.com/facebook/zstd/issues/1964

Best Regards,
Mingcong Bai