Re: [PATCH v11 20/26] virt: gunyah: Add resource tickets

[Elliot Berman <quic_eberman@quicinc.com>]

On 3/31/2023 7:27 AM, Alex Elder wrote:
> On 3/3/23 7:06 PM, Elliot Berman wrote:
>> Some VM functions need to acquire Gunyah resources. For instance, Gunyah
>> vCPUs are exposed to the host as a resource. The Gunyah vCPU function
>> will register a resource ticket and be able to interact with the
>> hypervisor once the resource ticket is filled.
>>
>> Resource tickets are the mechanism for functions to acquire ownership of
>> Gunyah resources. Gunyah functions can be created before the VM's
>> resources are created and made available to Linux. A resource ticket
>> identifies a type of resource and a label of a resource which the ticket
>> holder is interested in.
>>
>> Resources are created by Gunyah as configured in the VM's devicetree
>> configuration. Gunyah doesn't process the label and that makes it
>> possible for userspace to create multiple resources with the same label.
>> Resource ticket owners need to be prepared for populate to be called
>> multiple times if userspace created multiple resources with the same
>> label.
>>
>> Signed-off-by: Elliot Berman <quic_eberman@quicinc.com>
> 
> One possibly substantive suggestion here, plus a couple suggestions
> to add or revise comments.
> 
>                      -Alex
> 
>> ---
>>   drivers/virt/gunyah/vm_mgr.c  | 112 +++++++++++++++++++++++++++++++++-
>>   drivers/virt/gunyah/vm_mgr.h  |   4 ++
>>   include/linux/gunyah_vm_mgr.h |  14 +++++
>>   3 files changed, 129 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/virt/gunyah/vm_mgr.c b/drivers/virt/gunyah/vm_mgr.c
>> index 88db011395ec..0269bcdaf692 100644
>> --- a/drivers/virt/gunyah/vm_mgr.c
>> +++ b/drivers/virt/gunyah/vm_mgr.c
>> @@ -165,6 +165,74 @@ static long gh_vm_rm_function(struct gh_vm *ghvm, 
>> struct gh_fn_desc *f)
>>       return r;
>>   }
>> +int gh_vm_add_resource_ticket(struct gh_vm *ghvm, struct 
>> gh_vm_resource_ticket *ticket)
>> +{
>> +    struct gh_vm_resource_ticket *iter;
>> +    struct gh_resource *ghrsc;
>> +    int ret = 0;
>> +
>> +    mutex_lock(&ghvm->resources_lock);
>> +    list_for_each_entry(iter, &ghvm->resource_tickets, list) {
>> +        if (iter->resource_type == ticket->resource_type && 
>> iter->label == ticket->label) {
>> +            ret = -EEXIST;
>> +            goto out;
>> +        }
>> +    }
>> +
>> +    if (!try_module_get(ticket->owner)) {
>> +        ret = -ENODEV;
>> +        goto out;
>> +    }
>> +
>> +    list_add(&ticket->list, &ghvm->resource_tickets);
>> +    INIT_LIST_HEAD(&ticket->resources);
>> +
>> +    list_for_each_entry(ghrsc, &ghvm->resources, list) {
>> +        if (ghrsc->type == ticket->resource_type && ghrsc->rm_label 
>> == ticket->label) {
>> +            if (!ticket->populate(ticket, ghrsc))
>> +                list_move(&ghrsc->list, &ticket->resources);
>> +        }
>> +    }
>> +out:
>> +    mutex_unlock(&ghvm->resources_lock);
>> +    return ret;
>> +}
>> +EXPORT_SYMBOL_GPL(gh_vm_add_resource_ticket);
>> +
>> +void gh_vm_remove_resource_ticket(struct gh_vm *ghvm, struct 
>> gh_vm_resource_ticket *ticket)
>> +{
>> +    struct gh_resource *ghrsc, *iter;
>> +
>> +    mutex_lock(&ghvm->resources_lock);
>> +    list_for_each_entry_safe(ghrsc, iter, &ticket->resources, list) {
>> +        ticket->unpopulate(ticket, ghrsc);
>> +        list_move(&ghrsc->list, &ghvm->resources);
>> +    }
>> +
>> +    module_put(ticket->owner);
>> +    list_del(&ticket->list);
>> +    mutex_unlock(&ghvm->resources_lock);
>> +}
>> +EXPORT_SYMBOL_GPL(gh_vm_remove_resource_ticket);
>> +
>> +static void gh_vm_add_resource(struct gh_vm *ghvm, struct gh_resource 
>> *ghrsc)
>> +{
>> +    struct gh_vm_resource_ticket *ticket;
>> +
>> +    mutex_lock(&ghvm->resources_lock);
>> +    list_for_each_entry(ticket, &ghvm->resource_tickets, list) {
>> +        if (ghrsc->type == ticket->resource_type && ghrsc->rm_label 
>> == ticket->label) {
>> +            if (!ticket->populate(ticket, ghrsc)) {
>> +                list_add(&ghrsc->list, &ticket->resources);
>> +                goto found;
>> +            }
> 
> I think the "goto found" belongs here, unconditionally.
> You disallow adding more than one ticket of a given type
> with the same label.  So you will never match another
> ticket once you've matched this one.
> 
> The populate function generally shouldn't fail.  I think
> it only fails if you find a duplicate, and again, I think
> you prevent that from happening.  (But if it does, you
> silently ignore it...)
> 

I agree with this suggestion, no need to waste continue checking other 
tickets once we find the match. I'll move the "goto found" line.

[snip]

Thanks,
Elliot