On Tue, Jan 15, 2019 at 11:15:51AM +0100, Paolo Bonzini wrote: > On 15/01/19 03:43, Sean Christopherson wrote: > >> - vmx->nested.cached_vmcs12 = kmalloc(VMCS12_SIZE, GFP_KERNEL); > >> + vmx->nested.cached_vmcs12 = kzalloc(VMCS12_SIZE, GFP_KERNEL); > >> if (!vmx->nested.cached_vmcs12) > >> goto out_cached_vmcs12; > > Obviously not your code, but why do we allocate VMCS12_SIZE instead of > > sizeof(struct vmcs12)? I get why we require userspace to reserve the > > full 4k, but I don't understand why KVM needs to allocate the reserved > > bytes internally. > > It's just cleaner and shorter code to copy everything in and out, > instead of having to explicitly zero the slack. Could you please clarify? I don't see code that copies everything in and out, but it depends on what you mean by "everything". In the context of this email exchange, I assumed that "everything" was "all 4k (VMCS12_SIZE)". But it looks to me like the code doesn't copy 4k in and out, but rather only ever copies sizeof(struct vmcs12) in and out. The copy_from_user and copy_to_user cases in nested.c use sizeof(*vmcs12), which is sizeof(struct vmcs12). So maybe can switch to allocating sizeof(struct vmcs12). Is this correct, or is there some other reason to allocate the larger size?
