From mboxrd@z Thu Jan  1 00:00:00 1970
From: Halil Pasic <pasic-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>
Subject: Re: [PATCH 2/2] virtio: let virtio use DMA API when guest RAM is
 protected
Date: Mon, 24 Feb 2020 19:59:28 +0100
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To: Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org>
Cc: linux-s390-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, Viktor Mihajlovski <mihajlov-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>, Janosch Frank <frankja-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>, "Michael S. Tsirkin" <mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Jason Wang <jasowang-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Cornelia Huck <cohuck-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>, Ram Pai <linuxram-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, virtualization-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Christian Borntraeger <borntraeger-tA70FqPdS9bQT0dZR+AlfA@public.gmane.org>, iommu-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, Michael Mueller <mimu-tEXmvtCZX7AybS5Ee8rs3A@public.gmane.org>, "Lendacky, Thomas" <Thomas.Lendacky-5C7GfCeVMHo@public.gmane.org>, David Gibson <david-xT8FGy+AXnRB3Ne2BGzF6laj5H9X9Tb+@public.gmane.org>, Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
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On Fri, 21 Feb 2020 17:36:45 +0100
Christoph Hellwig <hch-jcswGhMUV9g@public.gmane.org> 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