Re: [Xen-devel] [RFC] xen/arm: Handling cache maintenance instructions by set/way

[Marc Zyngier <marc.zyngier@xxxxxxx>]

On 07/12/17 16:44, George Dunlap wrote:
> On 12/07/2017 04:04 PM, Julien Grall wrote:
>> Hi Jan,
>>
>> On 07/12/17 15:45, Jan Beulich wrote:
>>>>>> On 07.12.17 at 15:53, <marc.zyngier@xxxxxxx> wrote:
>>>> On 07/12/17 13:52, Julien Grall wrote:
>>>> There is exactly one case where set/way makes sense, and that's when
>>>> you're the only CPU left in the system, your MMU is off, and you're
>>>> about to go down.
>>>
>>> With this and ...
>>>
>>>> On top of bypassing the coherency, S/W CMOs do not prevent lines from
>>>> migrating from one CPU to another. So you could happily be flushing by
>>>> S/W, and still end up with dirty lines in your cache. Success!
>>>
>>> ... this I wonder what value emulating those insns then has in the first
>>> place. Can't you as well simply skip and ignore them, with the same
>>> (bad) result?
>>
>> The result will be much much worst. Here a concrete example with a Linux
>> Arm 32-bit:
>>
>>     1) Cache enabled
>>     2) Decompress
>>     3) Nuke cache (S/W)
>>     4) Cache off
>>     5) Access new kernel
>>
>> If you skip #3, the decompress data may not have reached the memory, so
>> you would access stall data.
>>
>> This would effectively mean we don't support Linux Arm 32-bit.
> 
> So Marc said that #3 "doesn't make sense", since although it might be
> the only cpu on in the system, you're not "about to go down"; but Linux
> 32-bit is doing that anyway.

"Doesn't make sense" on an ARMv7+ with SMP. That code dates back to
ARMv4, and has been left untouched ever since. "If it ain't broke..."

> It sounds like from the slides the purpose of #3 might be to get stuff
> out of the D-cache into the I-cache.  But why is the cache turned off?

Linux mandates that the kernel in entered with the MMU off. Which has
the effect of disabling the caches too (VIVT caches and all that jazz).

> And why doesn't Linux use the VA-based flushes rather than the S/W flushes?

Linux/arm64 does. Changing the 32bit port to use VA CMOs would probably
break stuff from the late 90s, so that's not going to happen. These
days, I tend to pick my battles... ;-)

        M.
-- 
Jazz is not dead. It just smells funny...

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