Re: [PATCH v2 2/3] KVM: x86: Allocate kvm_vmx/kvm_svm structures using kzalloc()

[Sean Christopherson <seanjc@google.com>]

On Tue, Apr 22, 2025, Kai Huang wrote:
> On Wed, 2025-04-16 at 12:57 -0700, Sean Christopherson wrote:
> > On Wed, Apr 16, 2025, Vipin Sharma wrote:
> > > Checked via pahole, sizes of struct have reduced but still not under 4k.
> > > After applying the patch:
> > > 
> > > struct kvm{} - 4104
> > > struct kvm_svm{} - 4320
> > > struct kvm_vmx{} - 4128
> > > 
> > > Also, this BUILD_BUG_ON() might not be reliable unless all of the ifdefs
> > > under kvm_[vmx|svm] and its children are enabled. Won't that be an
> > > issue?
> > 
> > That's what build bots (and to a lesser extent, maintainers) are for.  An individual
> > developer might miss a particular config, but the build bots that run allyesconfig
> > will very quickly detect the issue, and then we fix it.
> > 
> > I also build what is effectively an "allkvmconfig" before officially applying
> > anything, so in general things like this shouldn't even make it to the bots.
> > 
> 
> Just want to understand the intention here:
> 
> What if someday a developer really needs to add some new field(s) to, lets say
> 'struct kvm_vmx', and that makes the size exceed 4K?

If it helps, here's the changelog I plan on posting for v3:
    
    Allocate VM structs via kvzalloc(), i.e. try to use a contiguous physical
    allocation before falling back to __vmalloc(), to avoid the overhead of
    establishing the virtual mappings.  The SVM and VMX (and TDX) structures
    are now just above 4096 bytes, i.e. are order-1 allocations, and will
    likely remain that way for quite some time.
    
    Add compile-time assertions in vendor code to ensure the size is an
    order-0 or order-1 allocation, i.e. to prevent unknowingly letting the
    size balloon in the future.  There's nothing fundamentally wrong with a
    larger kvm_{svm,vmx,tdx} size, but given that the size is barely above
    4096 after 18+ years of existence, exceeding exceed 8192 bytes would be
    quite notable.


> What should the developer do?  Is it a hard requirement that the size should
> never go beyond 4K?  Or, should the assert of order 0 allocation be changed to
> the assert of order 1 allocation?

It depends.  Now that Vipin has corrected my math, the assertion will be that the
VM struct is order-1 or smaller, i.e. <= 8KiB.  That gives us a _lot_ of room to
grow.  E.g. KVM has existed for ~18 years and is barely about 4KiB, so for organic
growth (small additions here and there), I don't expect to hit the 8KiB limit in
the next decade (famous last words).  And the memory landscape will likely be
quite different 10+ years from now, i.e. the assertion may be completely unnecessary
by the time it fires.

What I'm most interested in detecting and prevent is things like mmu_page_hash,
where a massive field is embedded in struct kvm for an *optional* feature.  I.e.
if a new feature adds a massive field, then it should probably be placed in a
separate, dynamically allocated structure.  And for those, it should be quite
obvious that a separate allocation is the way to go.