Re: [PATCH v4 3/4] KVM: x86: Use kvzalloc() to allocate VM struct

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 5/23/2025 8:11 AM, Sean Christopherson wrote:
> 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.  For non-debug builds, The SVM and VMX
> (and TDX) structures are now just below 7000 bytes in the worst case
> scenario (see below), 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 of the
> structures, sans the memslos hash tables, are order-0 allocations, i.e.

s/memslos/memslots

> are less than 4KiB.  There's nothing fundamentally wrong with a larger
> kvm_{svm,vmx,tdx} size, but given that the size of the structure (without
> the memslots hash tables) is below 2KiB after 18+ years of existence,
> more than doubling the size would be quite notable.
> 
> Add sanity checks on the memslot hash table sizes, partly to ensure they
> aren't resized without accounting for the impact on VM structure size, and
> partly to document that the majority of the size of VM structures comes
> from the memslots.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>