Re: [Qemu-devel] [RFC/RFT PATCH v2 0/3] KVM: Introduce KVM_MEM_UNCACHED

[Christoffer Dall]

On Thu, May 14, 2015 at 01:09:34PM +0200, Laszlo Ersek wrote:
> On 05/14/15 12:30, Christoffer Dall wrote:
> > On Wed, May 13, 2015 at 01:31:51PM +0200, Andrew Jones wrote:
> >> Introduce a new memory region flag, KVM_MEM_UNCACHED, which is
> >> needed by ARM. This flag informs KVM that the given memory region
> >> is typically mapped by the guest as non-cacheable. KVM for ARM
> >> then ensures that that memory is indeed mapped non-cacheable by
> >> the guest, and also remaps that region as non-cacheable for
> >> userspace, allowing them both to maintain a coherent view.
> >>
> >> Changes since v1:
> >>  1) don't pin pages [Paolo]
> >>  2) ensure the guest maps the memory non-cacheable [me]
> >>  3) clean up memslot flag documentation [Christoffer]
> >> changes 1 and 2 effectively redesigned/rewrote v1. Find v1 here
> >> http://www.spinics.net/lists/kvm-arm/msg14022.html
> >>
> >> The QEMU series for v1 hasn't really changed. Only the linux
> >> header hack needed to bump KVM_CAP_UNCACHED_MEM from 107 to
> >> 116.  Find the series here
> >> http://www.spinics.net/lists/kvm-arm/msg14026.html
> >>
> >> Testing:
> >> This series still needs lots of testing, but I thought I'd
> >> kick it to the list early, as there's been recent interest
> >> in solving this problem, and I'd like to get test results
> >> and opinions on this approach from others sooner than later.
> >> I've tested with AAVMF (UEFI for AArch64 mach-virt guests).
> >> AAVMF has a kludge in it to avoid the coherency problem.
> > 
> > How does the 'kludge' work?
> 
> https://github.com/tianocore/edk2/commit/f9a8be42
> 
> (It's probably worth looking at the documentation in the first hunk too,
> under the commit message.)
> 
Why is this a hack/unintuitive?  Is the semantics of the QEMU PCI bus
not simply that MMIO regions are coherent?

-Christoffer