Re: [PATCH v16 03/20] KVM: selftests: Initialize the TDX VM

[Sean Christopherson]

On Mon, Nov 09, 2026, Ackerley Tng wrote:
> Lisa Wang <wyihan@xxxxxxxxxx> writes:
> > +static const struct kvm_tdx_capabilities *tdx_read_capabilities(struct kvm_vm *vm)
> > +{
> > +	static struct kvm_tdx_capabilities *tdx_cap;
> > +	int nr_cpuid_configs = 4;
> > +	int rc = -1;
> > +	int i;
> > +
> > +	if (tdx_cap)
> > +		return tdx_cap;
> > +
> > +	do {
> > +		nr_cpuid_configs *= 2;
> > +
> > +		tdx_cap = realloc(tdx_cap, sizeof(*tdx_cap) +
> > +					   (sizeof(struct kvm_cpuid_entry2) * nr_cpuid_configs));
> 
> Would be nice if struct_size() could be used here instead of manual
> calculation of the size. Same below.

...

> > +void tdx_init_vm(struct kvm_vm *vm)
> > +{
> > +	struct kvm_tdx_init_vm *init_vm;
> > +	const struct kvm_cpuid2 *tmp;
> > +
> > +	tmp = kvm_get_supported_cpuid();
> > +
> > +	init_vm = calloc(1, sizeof(*init_vm) +
> > +			 sizeof(init_vm->cpuid.entries[0]) * tmp->nent);
> 
> Would it be possible to use struct_size() here?
> Or perhaps kvm_cpuid2_size(tmp->nent)?
> 
> > +	TEST_ASSERT(init_vm, "init_vm allocation failed");
> > +
> > +	memcpy(&init_vm->cpuid, tmp, kvm_cpuid2_size(tmp->nent));
> > +	init_vm->cpuid.padding = 0;
> > +	tdx_filter_cpuid(vm, &init_vm->cpuid);
> 
> How about something like this?
> 
> 1. Query TDX capabilities first to get tdx_cap->cpuid.nent (let's say N).
> 2. Allocate init_vm sized for at most N entries, since the filtered
>    result can never exceed TDX's supported CPUID set.
> 3. Iterate over the TDX CPUID capabilities, look each one up in
>    kvm_get_supported_cpuid(), and append matching, masked entries
>    directly into init_vm->cpuid.entries.
> 
> This would avoid the memcpy and all the memmoves, though this works
> as-is. Just an optimization/different perspective.

At this point, I'll do minimial fixup when applying, e.g. if switching to
struct_size() Just Works.  But I don't want to do anything more invasive at this
point so that we can get TDX selftests landed asap.  To be clear, I'm not opposed
to more cleanups, I just want to do them on top.