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

[Christoffer Dall <christoffer.dall@linaro.org>]

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?

> I've tested both with and without that kludge active. Both
> worked for me (almost). Sometimes with the non-kludged
> version I was still able to see a bit of corruption in
> grub's output after edk2 loaded it - not much, and not always,
> but something.

Remind me, this is a VGA framebuffer corruption with a PCI-plugged VGA
card?

Thanks,
-Christoffer

> Anyway, it's quite frustrating, as I'm not sure
> what I'm missing...
> 
> This series applies to Linus' 110bc76729d4, but I tested with
> a version backported to the current RHELSA kernel.
> 
> Thanks for reviews and testing!
> 
> drew
> 
> 
> Andrew Jones (3):
>   arm/arm64: pageattr: add set_memory_nc
>   KVM: promote KVM_MEMSLOT_INCOHERENT to uapi
>   arm/arm64: KVM: implement 'uncached' mem coherency
> 
>  Documentation/virtual/kvm/api.txt     | 20 ++++++++++++------
>  arch/arm/include/asm/cacheflush.h     |  1 +
>  arch/arm/include/asm/kvm_mmu.h        |  5 ++++-
>  arch/arm/include/asm/pgtable-3level.h |  1 +
>  arch/arm/include/asm/pgtable.h        |  1 +
>  arch/arm/include/uapi/asm/kvm.h       |  1 +
>  arch/arm/kvm/arm.c                    |  1 +
>  arch/arm/kvm/mmu.c                    | 39 ++++++++++++++++++++++-------------
>  arch/arm/mm/pageattr.c                |  7 +++++++
>  arch/arm64/include/asm/cacheflush.h   |  1 +
>  arch/arm64/include/asm/kvm_mmu.h      |  5 ++++-
>  arch/arm64/include/asm/memory.h       |  1 +
>  arch/arm64/include/asm/pgtable.h      |  1 +
>  arch/arm64/include/uapi/asm/kvm.h     |  1 +
>  arch/arm64/mm/pageattr.c              |  8 +++++++
>  include/linux/kvm_host.h              |  1 -
>  include/uapi/linux/kvm.h              |  2 ++
>  virt/kvm/kvm_main.c                   |  7 ++++++-
>  18 files changed, 79 insertions(+), 24 deletions(-)
> 
> -- 
> 2.1.0
>