[RFC] ARM/ARM64: KVM: Implement KVM_FLUSH_DCACHE_GPA ioctl

[Christoffer Dall]

On Thu, May 07, 2015 at 12:50:50PM +0200, Jérémy Fanguède wrote:
> On Wed, May 6, 2015 at 4:12 PM, Christoffer Dall
> <christoffer.dall at linaro.org> wrote:
> > Hi Jérémy,
> >
> > On Tue, May 05, 2015 at 11:13:11AM +0200, Jérémy Fanguède wrote:
> >> To maintain cache coherency on ARM, we may need a mechanism to flush
> >> the data cache.
> >
> > In addition to generally just making this functionality available (see
> > below), do you have an actual use case in mind for this?  To solve the
> > VGA issue, for example, we already have a patch series from Drew trying
> > to address this.  Does that not work for you?
> >
> > There was a long discussion about this here:
> > https://lists.cs.columbia.edu/pipermail/kvmarm/2015-February/013593.html
> >
> > Drew then created a patch set, here:
> > https://lists.nongnu.org/archive/html/qemu-devel/2015-03/msg01254.html
> >
> > and replied to himself, here:
> > https://www.marc.info/?l=android-virt&m=142670523929132&w=3
> >
> > Which basically says that he doesn't like having to do flushes all over
> > QEMU (IIUC), so he sent this version instead:
> > https://lists.cs.columbia.edu/pipermail/kvmarm/2015-March/014027.html
> >
> > Which he now said he'd respin.
> 
> In fact, I used this ioctl in pairs with this QEMU patch series:
> https://lists.nongnu.org/archive/html/qemu-devel/2015-05/msg00407.html
> My current work doesn't do anything about vga ram, so vga issue
> probably still persists, but it solves others issues with some
> emulated devices (mainly PCI) which were failing before and now work
> fine with this patch.

Why does Drew's approach not work and your approach works here?  What is
the case that we haven't though about yet?

-Christoffer