On 9/27/22 19:07, Sean Christopherson wrote:
On Mon, Sep 26, 2022, Paolo Bonzini wrote:
32-bit KVM has extra complications in the code due to:
- different ways to write 64-bit values in VMCS
- different handling of DS and ES selectors as well as FS/GS bases
- lack of CR8 and EFER
- lack of XFD
More for the list:
- SVM is effectively restricted to PAE kernels due to NX requirements
True, but I'm not sure how it complicates the code? Do you mean having
to kmap/kunmap, and if so are you thinking of making KVM depend on !HIGHMEM?
- impossibility of writing 64-bit PTEs atomically
It's not impossible, just ugly. KVM could use CMPXCHG8B to do all of the accesses
for the TDP MMU, including the non-atomic reads and writes.
Ok, rephrased:
==========
Breakage in 32-bit KVM is somewhat rare, but it did happen and
developers themselves found out a few months later. This means that it
is very unlikely that it has any users. 32-bit processors with
virtualization extensions are a historical curiosity; 32-bit userspace
can only deal with small guests due to limited address space.
Hence, it makes sense to restrict x86 KVM to 64-bit hosts and kernels.
This removes all the conditional code to deal with the above
differences, and it also enables unconditional use of the TDP MMU:
making it work on 32-bit systems would require contortions using
CMPXCHG8B to write 64-bit PTEs atomically, and they are simply not
worth it.
==========
The last is the big one, because it prevents from using the TDP MMU
unconditionally.
As above, if the TDP MMU really is the sticking point, that's solvable.
Yeah, but until now the maintainability cost of keeping 32-bit on life
support was small. Using CMPXCHG8B in the TDP MMU is possibly worse
than having two MMUs for the two-dimensional paging case, therefore
dropping the old x86 direct MMU for me is what tips the balance in favor
of removal.
(Once David submits his MMU callbacks work, I still want to see if it's
possible to preserve the "old" x86 direct MMU, for example for use in
pKVM; however, it would not be in arch/x86/).
Paolo
The real justification for deprecating 32-bit KVM is that, outside of KVM developers,
literally no one uses 32-bit KVM. I.e. any amount of effort that is required to
continue supporting 32-bit kernels is a complete waste of resources.
