linux-kernel - Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is protected

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Message-ID: <20200224171657.GB7278@lst.de>
Date:   Mon, 24 Feb 2020 18:16:57 +0100
From:   Christoph Hellwig <hch@....de>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Halil Pasic <pasic@...ux.ibm.com>, Christoph Hellwig <hch@....de>,
        Jason Wang <jasowang@...hat.com>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Robin Murphy <robin.murphy@....com>,
        linux-s390@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux-foundation.org,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Viktor Mihajlovski <mihajlov@...ux.ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        Ram Pai <linuxram@...ibm.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        David Gibson <david@...son.dropbear.id.au>,
        "Lendacky, Thomas" <Thomas.Lendacky@....com>,
        Michael Mueller <mimu@...ux.ibm.com>
Subject: Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is
 protected

On Sat, Feb 22, 2020 at 02:07:58PM -0500, Michael S. Tsirkin wrote:
> On Fri, Feb 21, 2020 at 03:33:40PM +0100, Halil Pasic wrote:
> > AFAIU you have a positive attitude towards the idea, that 
> > !F_VIRTIO_PLATFORM implies 'no DMA API is used by virtio' 
> > should be scrapped. 
> > 
> > I would like to accomplish that without adverse effects to virtio-ccw
> > (because caring for virtio-ccw is a part of job description).
> > 
> > Regards,
> > Halil
> 
> It is possible, in theory. IIRC the main challenge is that DMA API
> has overhead of indirect function calls even when all it
> does it return back the PA without changes.

That overhead is gone now, the DMA direct calls are direct calls these
days.