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

From: Peter Maydell
Subject: Re: [Qemu-devel] [RFC/RFT PATCH v2 0/3] KVM: Introduce KVM_MEM_UNCACHED
Date: Thu, 14 May 2015 14:11:59 +0100

On 14 May 2015 at 14:03, Andrew Jones <address@hidden> wrote:
> On Thu, May 14, 2015 at 11:37:46AM +0100, Peter Maydell wrote:
>> On 14 May 2015 at 11:31, Andrew Jones <address@hidden> 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 rather than merely
hoping NormalNC is OK. (For instance, do we need to provide
support for QEMU to map both NC and writethrough?)

-- PMM

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