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

[Laszlo Ersek]

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.)

Thanks
Laszlo


> 
>> 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
>>