Re: Virtio in Xen on Arm (based on IOREQ concept)

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Wed, Jul 22, 2020 at 12:17:26PM +0100, Julien Grall wrote:
> 
> 
> On 22/07/2020 12:10, Roger Pau Monné wrote:
> > On Wed, Jul 22, 2020 at 11:47:18AM +0100, Julien Grall wrote:
> > > > 
> > > > You can still use the map-on-fault behaviour as above, but I would
> > > > recommend that you try to limit the number of hypercalls issued.
> > > > Having to issue a single hypercall for each page fault it's going to
> > > > be slow, so I would instead use mmap batch to map the hole range in
> > > > unpopulated physical memory and then the OS fault handler just needs to
> > > > fill the page tables with the corresponding address.
> > > IIUC your proposal, you are assuming that you will have enough free space 
> > > in
> > > the physical address space to map the foreign mapping.
> > > 
> > > However that amount of free space is not unlimited and may be quite small
> > > (see above). It would be fairly easy to exhaust it given that a userspace
> > > application can map many times the same guest physical address.
> > > 
> > > So I still think we need to be able to allow Linux to swap a foreign page
> > > with another page.
> > 
> > Right, but you will have to be careful to make sure physical addresses
> > are not swapped while being used for IO with devices, as in that case
> > you won't get a recoverable fault. This is safe now because physical
> > mappings created by privcmd are never swapped out, but if you go the
> > route you propose you will have to figure a way to correctly populate
> > physical ranges used for IO with devices, even when the CPU hasn't
> > accessed them.
> > 
> > Relying solely on CPU page faults to populate them will not be enough,
> > as the CPU won't necessarily access all the pages that would be send
> > to devices for IO.
> 
> The problem you described here doesn't seem to be specific to foreign
> mapping. So I would really be surprised if Linux doesn't already have
> generic mechanism to deal with this.

Right, Linux will pre-fault and lock the pages before using them for
IO, and unlock them afterwards, in which case it should be safe.

> Hence why I suggested before to deal with foreign mapping the same way as
> Linux would do with user memory.

Should work, on FreeBSD privcmd I also populate the pages in the page
fault handler, but the hypercall to create the foreign mappings is
executed only once when the ioctl is issued.

Roger.