Re: i386 kexec-tools for x86_64 kdump kernels

[Dave Young <dyoung@redhat.com>]

Hi Kevin,
On 05/17/21 at 09:40pm, Kevin Mitchell wrote:
> Hi,
> 
> As a space-saving strategy for our embedded boot environment, we use an i386
> kexec binary to load our x86_64 kdump kernel from an x86_64 system kernel. This
> worked great up until linux-5.2, which included the commit
> 
> 9ca5c8e632ce ("x86/kdump: Have crashkernel=X reserve under 4G by
> default")
> 
> Sure enough, according to /proc/iomem, the "Crash kernel" area went from
> starting at 0x34000000 to 0x7b000000, which is above the 896M
> limit. Unfortunately, since i386 kexec seems to use
> kexec/arch/i386/kexec-bzImage.c even to load an x86_64 kernel, the
> DEFAULT_BZIMAGE_ADDR_MAX = 0x37FFFFFF 896M limit is still enforced when loading
> the panic kernel:
> 
> # kexec32 --load-panic bzImage64
> Could not find a free area of memory of 0x8000 bytes...
> locate_hole failed
> 
> I can work around this by patching kexec-tools to raise that limit to
> DEFAULT_BZIMAGE_ADDR_MAX = 0xFFFFFFFF which allows loading the x86_64 kdump
> bzImage. This does in fact kexec fine from that position if I trigger a panic.
> 
> However, this doesn't appear to be a general solution since the 896M does still
> apply if either of the kernels is i386. In that case, attempting to kexec from
> the higher address will just hang with no console output. In this case, it
> probably is better to continue to fail to load the kdump image rather than wait
> until the panic to find out something is wrong.

I'm not sure if you can try to detect the kernel type and special case
this in kexec-tools, eg. if the 2nd kernel is 64-bit kernel then just
bump the addr max otherwise go original logic.  If this is doable then
it would be a good way IMO.

See if Eric, Baoquan and other X86 people have more idea.

> 
> Fortunately, while 9ca5c8e632ce allows an i386 kernel to reserve a "Crash
> kernel" region > 896M, it doesn't actually do that by default - I have to force
> it to go there with crashkernel=@. I am not sure if this is just a fluke or if
> there is something actually ensuring it defaults to a working
> location. Nevertheless, it appears the restriction removed by this commit is
> still required by i386 kernels. Its enforcement has just moved to userspace.
> 
> So it seems that the largest fallout of the commit is restricted to the
> admittedly niche combination linux-x86_64 -> kexec-i386 -> linux-x86_64(kdump),
> which no longer works out of the box without pinning the crashkernel address or
> patching kexec.
> 
> Is this just something we need to live with or is it worth looking into how to
> better support this combination?

This is the case I missed, but I would think it as not a common use
case. It would be better to leave it as is in kernel and try to fix in
kexec-tools or just use the workaround.

> 
> Thanks,
> Kevin
> 

Thanks
Dave