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

[Laszlo Ersek <lersek@redhat.com>]

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
>