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

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Message-Id: <0c508eb2-08df-3f76-c260-90cf7137af80@linux.vnet.ibm.com>
Date:   Thu, 31 May 2018 09:09:24 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     Ram Pai <linuxram@...ibm.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     robh@...nel.org, aik@...abs.ru, jasowang@...hat.com,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, hch@...radead.org,
        joe@...ches.com, linuxppc-dev@...ts.ozlabs.org,
        elfring@...rs.sourceforge.net, david@...son.dropbear.id.au
Subject: Re: [RFC V2] virtio: Add platform specific DMA API translation for
 virito devices

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.