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

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Date:   Thu, 31 May 2018 20:43:58 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Anshuman Khandual <khandual@...ux.vnet.ibm.com>
Cc:     Ram Pai <linuxram@...ibm.com>, 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 Thu, May 31, 2018 at 09:09:24AM +0530, Anshuman Khandual wrote:
> 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.
> 

Pls work on a long term solution. Short term needs can be served by
enabling the iommu platform in qemu.

-- 
MST