linux-kernel - Re: [PATCH v5 09/13] xen/pvcalls: implement sendmsg

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Date:   Tue, 17 Oct 2017 17:06:32 -0400
From:   Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org
Cc:     linux-kernel@...r.kernel.org, jgross@...e.com,
        Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v5 09/13] xen/pvcalls: implement sendmsg


> +static int __write_ring(struct pvcalls_data_intf *intf,
> +			struct pvcalls_data *data,
> +			struct iov_iter *msg_iter,
> +			int len)
> +{
> +	RING_IDX cons, prod, size, masked_prod, masked_cons;
> +	RING_IDX array_size = XEN_FLEX_RING_SIZE(PVCALLS_RING_ORDER);
> +	int32_t error;
> +
> +	error = intf->out_error;
> +	if (error < 0)
> +		return error;
> +	cons = intf->out_cons;
> +	prod = intf->out_prod;
> +	/* read indexes before continuing */
> +	virt_mb();
> +
> +	size = pvcalls_queued(prod, cons, array_size);
> +	if (size >= array_size)
> +		return 0;


I thought you were going to return an error here? If this can only be
due to someone messing up indexes is there a reason to continue trying
to write? What are the chances that the index will get corrected?

-boris

> +	if (len > array_size - size)
> +		len = array_size - size;
> +
> +	masked_prod = pvcalls_mask(prod, array_size);
> +	masked_cons = pvcalls_mask(cons, array_size);
> +
> +	if (masked_prod < masked_cons) {
> +		copy_from_iter(data->out + masked_prod, len, msg_iter);
> +	} else {
> +		if (len > array_size - masked_prod) {
> +			copy_from_iter(data->out + masked_prod,
> +				       array_size - masked_prod, msg_iter);
> +			copy_from_iter(data->out,
> +				       len - (array_size - masked_prod),
> +				       msg_iter);
> +		} else {
> +			copy_from_iter(data->out + masked_prod, len, msg_iter);
> +		}
> +	}
> +	/* write to ring before updating pointer */
> +	virt_wmb();
> +	intf->out_prod += len;
> +
> +	return len;
> +}