Re: [RFC 5/5] x86: kvm drop 32-bit host support

["Arnd Bergmann" <arnd@arndb.de>]

>n Thu, Dec 12, 2024, at 17:27, Paolo Bonzini wrote:
> On 12/12/24 13:55, Arnd Bergmann wrote:
>> From: Arnd Bergmann <arnd@arndb.de>
>> 
>> There are very few 32-bit machines that support KVM, the main exceptions
>> are the "Yonah" Generation Xeon-LV and Core Duo from 2006 and the Atom
>> Z5xx "Silverthorne" from 2008 that were all released just before their
>> 64-bit counterparts.
>
> Unlike other architectures where you can't run a "short bitness" kernel 
> at all, or 32-bit systems require hardware enablement that simply does 
> not exist, the x86 situation is a bit different: 32-bit KVM would not be 
> used on 32-bit processors, but on 64-bit kernels running 32-bit kernels; 
> presumably on a machine with 4 or 8 GB of memory, above which you're 
> hurting yourself even more, and for smaller guests where the limitations 
> in userspace address space size don't matter.
>
> Apart from a bunch of CONFIG_X86_64 conditionals, the main issue that 
> KVM has with 32-bit x86 is that they cannot read/write a PTE atomically 
> (i.e. without tearing) and therefore they can't use the newer and more 
> scalable page table management code.  So no objections from me for 
> removing this support, but the justification should be the truth, i.e. 
> developers don't care enough.

Right, I should have updated the description based on the comments
for the first version, especially after separating it from the patches
that make it harder to run 32-bit kernels on 64-bit hardware.

I've updated the changelog now to

    x86: kvm drop 32-bit host support
    
    There are very few 32-bit machines that support KVM, the main exceptions
    are the "Yonah" Generation Xeon-LV and Core Duo from 2006 and the Atom
    Z5xx "Silverthorne" from 2008 that were all released just before their
    64-bit counterparts.
    
    The main usecase for KVM in x86-32 kernels these days is to verify
    that 32-bit KVM is still working, by running it on 64-bit hardware.
    With KVM support on other 32-bit architectures going away, and x86-32
    kernels on 64-bit hardware becoming more limited in available RAM,
    this usecase becomes much less interesting.
    
    Remove this support to make KVM exclusive to 64-bit hosts on all
    architectures, and stop testing 32-bit host mode.
    
    Link: https://lore.kernel.org/all/Z1B1phcpbiYWLgCD@google.com/
    Signed-off-by: Arnd Bergmann <arnd@arndb.de>

which assumes that we end up going ahead with the powerpc
patches. Does that work for you?

      Arnd