Re: [PATCH 19/38] KVM: PPC: Add cache flush on page map

[Scott Wood <scottwood@freescale.com>]

On 08/15/2012 01:01 PM, Alexander Graf wrote:
> 
> On 15.08.2012, at 19:47, Scott Wood wrote:
> 
>> On 08/15/2012 12:27 PM, Alexander Graf wrote:
>>>
>>> On 15.08.2012, at 19:26, Scott Wood wrote:
>>>
>>>> On 08/15/2012 04:52 AM, Alexander Graf wrote:
>>>>>
>>>>> On 15.08.2012, at 03:23, Scott Wood wrote:
>>>>>
>>>>>> On 08/14/2012 06:04 PM, Alexander Graf wrote:
>>>>>>> When we map a page that wasn't icache cleared before, do so when first
>>>>>>> mapping it in KVM using the same information bits as the Linux mapping
>>>>>>> logic. That way we are 100% sure that any page we map does not have stale
>>>>>>> entries in the icache.
>>>>>>
>>>>>> We're not really 100% sure of that -- this only handles the case where
>>>>>> the kernel does the dirtying, not when it's done by QEMU or the guest.
>>>>>
>>>>> When the guest does it, the guest is responsible for clearing the
>>>>> icache. Same for QEMU. It needs to clear it when doing DMA.
>>>>
>>>> Sure.  I was just worried that that commit message could be taken the
>>>> wrong way, as in "we no longer need the QEMU icache flushing patch".
>>>>
>>>>> However, what is still broken would be a direct /dev/mem map. There
>>>>> QEMU should probably clear the icache before starting the guest, in
>>>>> case another guest was running on that same memory before.
>>>>> Fortunately, we don't have that mode available in upstream QEMU :).
>>>>
>>>> How is QEMU loading images different if it's /dev/mem versus ordinary
>>>> anonymous memory?  You probably won't have stale icache data in the
>>>> latter case (which makes it less likely to be a problem in pratice), but
>>>> in theory you could have data that still hasn't left the dcache.
>>>
>>> It's the same. I just talked to Ben about this today in a different context and we should be safe :).
>>
>> Safe how?
>>
>> If it's truly the same, we're definitely not safe, since I had problems
>> with this using /dev/mem (particularly when changing the kernel image
>> without a host reboot) before I put in the icache flush patch.
> 
> QEMU needs to icache flush everything it puts into guest memory.

Yes.  I thought you meant we should be safe as things are now.

-Scott