Re: [PATCH v5 8/8] KVM: VMX: Resize PID-ponter table on demand for IPI virtualization

[Sean Christopherson <seanjc@google.com>]

On Fri, Dec 31, 2021, Zeng Guang wrote:
> +static int vmx_expand_pid_table(struct kvm_vmx *kvm_vmx, int entry_idx)
> +{
> +	u64 *last_pid_table;
> +	int last_table_size, new_order;
> +
> +	if (entry_idx <= kvm_vmx->pid_last_index)
> +		return 0;
> +
> +	last_pid_table = kvm_vmx->pid_table;
> +	last_table_size = table_index_to_size(kvm_vmx->pid_last_index + 1);
> +	new_order = get_order(table_index_to_size(entry_idx + 1));
> +
> +	if (vmx_alloc_pid_table(kvm_vmx, new_order))
> +		return -ENOMEM;
> +
> +	memcpy(kvm_vmx->pid_table, last_pid_table, last_table_size);
> +	kvm_make_all_cpus_request(&kvm_vmx->kvm, KVM_REQ_PID_TABLE_UPDATE);
> +
> +	/* Now old PID table can be freed safely as no vCPU is using it. */
> +	free_pages((unsigned long)last_pid_table, get_order(last_table_size));

This is terrifying.  I think it's safe?  But it's still terrifying.

Rather than dynamically react as vCPUs are created, what about we make max_vcpus
common[*], extend KVM_CAP_MAX_VCPUS to allow userspace to override max_vcpus,
and then have the IPIv support allocate the PID table on first vCPU creation
instead of in vmx_vm_init()?

That will give userspace an opportunity to lower max_vcpus to reduce memory
consumption without needing to dynamically muck with the table in KVM.  Then
this entire patch goes away.