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

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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. Whatever blob comes next (SLOF for -M pseries, kernel for -M e500) assumes dirty icache for every page it maps itself. So SLOF will icache flush the kernel text segment, Linux will icache flush user space pages.


Alex

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