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

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Message-Id: <20200224195928.654694fd.pasic@linux.ibm.com>
Date:   Mon, 24 Feb 2020 19:59:28 +0100
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     David Gibson <david@...son.dropbear.id.au>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        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>,
        "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 Fri, 21 Feb 2020 17:36:45 +0100
Christoph Hellwig <hch@....de> wrote:

> > By "legacy devices" I assume you mean pre-virtio-1.0 devices, that
> > lack the F_VERSION_1 feature flag.  Is that right?  Because I don't
> > see how being a legacy device or not relates to use of the DMA API.  
> 
> No.   "legacy" is anything that does not set F_ACCESS_PLATFORM.

FYI in virtio-speak the term 'legacy devices' is already taken and it
ain't congruent with your intended semantics. Please look it up
https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-60001

But with that understood your statement does provide insisting in how do
you see F_ACCESS_PLATFORM. I.e. something that should be enabled in
general, except for legacy reasons. But I'm afraid Michael sees it
differently: i.e. something that should be enabled when necessary, and
otherwise not (because it is likely to cost performance).

Regards,
Halil