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.
