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Date: Thu, 14 May 2015 15:33:57 +0200
From: Laszlo Ersek <lersek@redhat.com>
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Subject: Re: [Qemu-devel] [RFC/RFT PATCH v2 0/3] KVM: Introduce
	KVM_MEM_UNCACHED
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To: Peter Maydell <peter.maydell@linaro.org>, Andrew Jones <drjones@redhat.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>, Marc Zyngier <marc.zyngier@arm.com>, Catalin Marinas <catalin.marinas@arm.com>, QEMU Developers <qemu-devel@nongnu.org>, Alexander Graf <agraf@suse.de>, Paolo Bonzini <pbonzini@redhat.com>, "kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>, Christoffer Dall <christoffer.dall@linaro.org>

On 05/14/15 15:11, Peter Maydell wrote:
> On 14 May 2015 at 14:03, Andrew Jones <drjones@redhat.com> wrote:
>> On Thu, May 14, 2015 at 11:37:46AM +0100, Peter Maydell wrote:
>>> On 14 May 2015 at 11:31, Andrew Jones <drjones@redhat.com> wrote:
>>>> Forgot to (4): switch from setting userspace's mapping to
>>>> device memory to normal, non-cacheable. Using device memory
>>>> caused a problem that Alex Graf found, and Peter Maydell suggested
>>>> using normal, non-cacheable instead.
>>>
>>> Did you check that non-cacheable is definitely the correct
>>> kind of Normal memory attribute we want? (ie not write-through).
>>
>> I was concerned that write-through wouldn't be sufficient. If the
>> guest writes to its non-cached memory, and QEMU needs to see what
>> it wrote, then won't write-through fail to work? Unless we some
>> how invalidate the cache first?
> 
> Well, I meant more that the correct mapping for userspace is
> the same as the guest, whatever that is, and so somebody needs
> to look at what the guest actually does

I think Ard explored this earlier, by tracking the MAIRs and stuff.
Can't recall what the findings were though. (Or I could be simply
confused, sorry.)

Laszlo

> rather than merely
> hoping NormalNC is OK. (For instance, do we need to provide
> support for QEMU to map both NC and writethrough?)
> 
> -- PMM
>