KVM: x86: Use kvzalloc() to allocate VM struct

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 memslot hash tables, are order-0 allocations, i.e.
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.

Reviewed-by: Xiaoyao Li <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Sean Christopherson <[email protected]>

1 files changed, 1 insertions, 1 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 17529e5d717c..109a5cfd68b6 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1962,7 +1962,7 @@ void kvm_x86_vendor_exit(void);
 #define __KVM_HAVE_ARCH_VM_ALLOC
 static inline struct kvm *kvm_arch_alloc_vm(void)
 {
-	return __vmalloc(kvm_x86_ops.vm_size, GFP_KERNEL_ACCOUNT | __GFP_ZERO);
+	return kvzalloc(kvm_x86_ops.vm_size, GFP_KERNEL_ACCOUNT);
 }
 
 #define __KVM_HAVE_ARCH_VM_FREE