[PATCH] ARM64: KVM: Fix coherent_icache_guest_page() for host with external L3-cache.

[anup@brainfault.org (Anup Patel)]

On Wed, Aug 14, 2013 at 9:27 PM, Marc Zyngier <maz@misterjones.org> wrote:
> On 2013-08-14 16:41, Peter Maydell wrote:
>> On 14 August 2013 16:34, Marc Zyngier <maz@misterjones.org> wrote:
>>> When userspace loads the kernel into memory, the kernel is not
>>> flushed
>>> to RAM, and may sit in the L3 cache if the cache is big enough. You
>>> end-up executing garbage... My proposed fix is to let kvmtool do the
>>> flushing, as we have userspace cache management operations for this
>>> exact purpose.
>>
>> Why does this issue only apply to the loaded kernel and not to
>> the zero bytes in the rest of RAM? I know executing zeroes
>> isn't a very useful thing to do but it should be a well defined
>> thing.
>
> Good point, and not quite sure just yet. Probably we get a zeroed,
> clean page?

This would apply to zeroed pages too if some Guest OS expects
zeroed-out portions in Guest RAM.

>
> Anup, can you elaborate on how your L3 cache behaves?

The L3-cache is transparent to Aarch64 CPUs. It is only bypassed
when caching is disabled or when accessing non-cacheable pages.
Currently, we cannot disclose more details about line allocation and
replacement policy because APM X-Gene specs are not released.

The issue pointed out by this patch would also apply to L2-cache
(i.e. L3-cache disabled) but it usually does not manifest because
L2-cache is not too big (few hundred KBs) and so dirty lines don't
stay for long time in L2-cache.

>
> Thanks,
>
>          M.
> --
> Who you jivin' with that Cosmik Debris?
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Regards,
Anup