Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices

From: Anshuman Khandual <khandual@linux.vnet.ibm.com>
To: Ram Pai <linuxram@us.ibm.com>, "Michael S. Tsirkin" <mst@redhat.com>
Cc: robh@kernel.org, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, hch@infradead.org,
	joe@perches.com, linuxppc-dev@lists.ozlabs.org,
	elfring@users.sourceforge.net, david@gibson.dropbear.id.au
Subject: Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices
Date: Thu, 31 May 2018 09:09:24 +0530	[thread overview]
Message-ID: <0c508eb2-08df-3f76-c260-90cf7137af80__31777.5298179642$1527737863$gmane$org@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180524072104.GD6139@ram.oc3035372033.ibm.com>

On 05/24/2018 12:51 PM, Ram Pai wrote:
> On Wed, May 23, 2018 at 09:50:02PM +0300, Michael S. Tsirkin wrote:
>> subj: s/virito/virtio/
>>
> ..snip..
>>>  machine_subsys_initcall_sync(pseries, tce_iommu_bus_notifier_init);
>>> +
>>> +bool platform_forces_virtio_dma(struct virtio_device *vdev)
>>> +{
>>> +	/*
>>> +	 * On protected guest platforms, force virtio core to use DMA
>>> +	 * MAP API for all virtio devices. But there can also be some
>>> +	 * exceptions for individual devices like virtio balloon.
>>> +	 */
>>> +	return (of_find_compatible_node(NULL, NULL, "ibm,ultravisor") != NULL);
>>> +}
>>
>> Isn't this kind of slow?  vring_use_dma_api is on
>> data path and supposed to be very fast.
> 
> Yes it is slow and not ideal. This won't be the final code. The final
> code will cache the information in some global variable and used
> in this function.

Right this will be a simple check based on a global variable.