Re: [PATCH v11 24/26] virt: gunyah: Add irqfd interface

[Elliot Berman]

On 3/31/2023 7:27 AM, Alex Elder wrote:
> On 3/3/23 7:06 PM, Elliot Berman wrote:

[snip]

> > +
> > +static int irqfd_wakeup(wait_queue_entry_t *wait, unsigned int mode, 
> > int sync, void *key)
> > +{
> > +    struct gh_irqfd *irqfd = container_of(wait, struct gh_irqfd, wait);
> > +    __poll_t flags = key_to_poll(key);
> > +    u64 enable_mask = GH_BELL_NONBLOCK;
> > +    u64 old_flags;
> > +    int ret = 0;
> > +
> > +    if (flags & EPOLLIN) {
> > +        if (irqfd->ghrsc) {
> > +            ret = gh_hypercall_bell_send(irqfd->ghrsc->capid, 
> > enable_mask, &old_flags);
>
> I commented elsewhere that you might support passing a null
> pointer as the last argument above (since you don't use the
> result).
>
> > +            if (ret)
> > +                pr_err_ratelimited("Failed to inject interrupt %d: 
> > %d\n",
> > +                        irqfd->ticket.label, ret);
> > +        } else
> > +            pr_err_ratelimited("Premature injection of interrupt\n");
> > +    }
> > +
> > +    return 0;
> > +}
> > +
> > +static void irqfd_ptable_queue_proc(struct file *file, 
> > wait_queue_head_t *wqh, poll_table *pt)
> > +{
> > +    struct gh_irqfd *irq_ctx = container_of(pt, struct gh_irqfd, pt);
> > +
> > +    add_wait_queue(wqh, &irq_ctx->wait);
> > +}
> > +
> > +static int gh_irqfd_populate(struct gh_vm_resource_ticket *ticket, 
> > struct gh_resource *ghrsc)
> > +{
> > +    struct gh_irqfd *irqfd = container_of(ticket, struct gh_irqfd, 
> > ticket);
> > +    u64 enable_mask = GH_BELL_NONBLOCK;
> > +    u64 ack_mask = ~0;
>
> Why is the ACK mask ~0?
>
> I guess I don't know details about this hypercall (do you document
> them somewhere?), so it's hard to judge whether or why this is the
> right thing to use.  The enable_mask is just GH_BELL_NONBLOCK,
> which is just BIT(32).

I talked to our hypervisor folks and they mentioned we can simplify 
this. In v12, enable_mask and ack_mask can just be "1" (BIT(0)). We had 
chosen bit 32 arbitrarily.

[snip]

> > +    }
> > +
> > +    irqfd->ghrsc = ghrsc;
> > +    if (irqfd->level) {
>
> I think I don't understand this part of the code well
> enough to know this.  What happens if level is false?

If level is false, then guest is assumed to set up IRQ on its side as 
edge-triggered. In that case, we don't need to configure the enable 
mask/ack mask because the doorbell flags aren't polled.

[snip]

> > +/**
> > + * struct gh_fn_irqfd_arg - Arguments to create an irqfd function
> > + * @fd: an eventfd which when written to will raise a doorbell
> > + * @label: Label of the doorbell created on the guest VM
> > + * @flags: GH_IRQFD_LEVEL configures the corresponding doorbell to 
> > behave
> > + *         like a level triggered interrupt.
> > + * @padding: padding bytes
> > + */
> > +struct gh_fn_irqfd_arg {
> > +    __u32 fd;
>
> Should the "fd" field be signed?  Should it be an int?  (Perhaps
> you're trying to define a fixed kernel API, so __s32 if signed would
> be better.)

It looked to me like some interfaces use __u32 and some use __s32. Is 
one technically correct?