[virtio-comment] Re: RFC: virtio-hostmem (+ Continuation of discussion from [virtio-dev] Memory sharing device)

From: Roman Kiryanov <rkir@google.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Frank Yang <lfy@google.com>,
	virtio-comment@lists.oasis-open.org,
	Cornelia Huck <cohuck@redhat.com>,
	Gerd Hoffmann <kraxel@redhat.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	"Dr. David Alan Gilbert" <dgilbert@redhat.com>
Subject: [virtio-comment] Re: RFC: virtio-hostmem (+ Continuation of discussion from [virtio-dev] Memory sharing device)
Date: Mon, 25 Feb 2019 10:54:03 -0800	[thread overview]
Message-ID: <CAOGAQeqGwzRiaqHQG3o3U19p_8S4R=RTjSyxTWqD-2fb677GBg@mail.gmail.com> (raw)
In-Reply-To: <20190225083700-mutt-send-email-mst@kernel.org>

> >   • The host does not back the region at all and a page fault happens.
>
> Then what? Guest dies?
> That doesn't sound reasonable, in particular if you want to
> allow userspace to map this memory.

In our implementation we call mmap after asking the host to back the region.

https://photos.app.goo.gl/NJvPBvvFS3S3n9mn6

Nothing prevents a guest to call mmap on an unbacked region, then the
guest will die. If it is possible for the device to figure out if an
address range
is backed in VM, the guest driver could talk to the device to fail an mmap
call if a region is not accessible.

> >   • The host has already allocated host RAM (from some source; vkMapMemory,
> >     malloc(), mmap, etc) memory of some kind and maps a page-aligned host
> >     pointer to the guest physical address corresponding to the region.
>
> I'm not sure what does "of some kind" mean here.

Memory from any API call that could be used for access through this
address range.

> Also host and guest might have different ideas about
> what does page-aligned mean.

In our implementation we do aligning (for VM operations) and unaligning in the
guest userspace (because mmap is page aligned) to get the pointer to handle
pointers in the middle of a page (we have no control on pointers returned
from a third party API).

Regards,
Roman.

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