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

[Jérémy Fanguède]

On Thu, May 7, 2015 at 1:20 PM, Christoffer Dall
<christoffer.dall at linaro.org> wrote:
> 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?

The first patch from Andrew, (which is for arm64 only) doesn't permit
me to make some emulated PCI devices working with virt, probably
because some flushes/cleans are missing.
As for the second patch, it focuses, for now, only on VGA ram. I
quickly tried to extend the KVM_MEM_UNCACHED flag to the whole guest
memory, but then the guest fails to boot; but even if it was working,
does it make sense to map as uncached all the ram of the guest? Since
we can not guess which region of the guest memory will be accessed.

Simple PCI devices like e1000 or usb-ehci (with usb-kbd for instance)
are not usable, with or without these patches, but if I flush a
precise memory range, on reads and writes performed by emulated
devices on guest ram, (It's exactly what the QEMU patch series that I
sent does, with this ioctl), emulated PCI devices are now working.