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

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Message-ID: <c8e95fd5-5761-b9aa-2877-6a8827a76f21@quicinc.com>
Date:   Mon, 17 Apr 2023 15:55:26 -0700
From:   Elliot Berman <quic_eberman@...cinc.com>
To:     Alex Elder <elder@...aro.org>,
        Srinivas Kandagatla <srinivas.kandagatla@...aro.org>,
        Prakruthi Deepak Heragu <quic_pheragu@...cinc.com>,
        Jonathan Corbet <corbet@....net>
CC:     Murali Nalajala <quic_mnalajal@...cinc.com>,
        Trilok Soni <quic_tsoni@...cinc.com>,
        Srivatsa Vaddagiri <quic_svaddagi@...cinc.com>,
        Carl van Schaik <quic_cvanscha@...cinc.com>,
        Dmitry Baryshkov <dmitry.baryshkov@...aro.org>,
        Bjorn Andersson <andersson@...nel.org>,
        "Konrad Dybcio" <konrad.dybcio@...aro.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
        Rob Herring <robh+dt@...nel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
        Bagas Sanjaya <bagasdotme@...il.com>,
        Will Deacon <will@...nel.org>, Andy Gross <agross@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Jassi Brar <jassisinghbrar@...il.com>,
        <linux-arm-msm@...r.kernel.org>, <devicetree@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v11 24/26] virt: gunyah: Add irqfd interface



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?