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

[Marc Zyngier]

On 2013-08-30 11:44, Anup Patel wrote:
> On Fri, Aug 30, 2013 at 3:22 PM, Catalin Marinas
> <catalin.marinas at arm.com> wrote:
>> On Fri, Aug 16, 2013 at 07:11:51PM +0100, Anup Patel wrote:
>>> On Fri, Aug 16, 2013 at 11:20 PM, Christoffer Dall
>>> <christoffer.dall at linaro.org> wrote:
>>> > On Fri, Aug 16, 2013 at 11:12:08PM +0530, Anup Patel wrote:
>>> >> Discussion here is about getting KVM ARM64 working in-presence
>>> >> of an external L3-cache (i.e. not part of CPU). Before starting 
>>> a VCPU
>>> >> user-space typically loads images to guest RAM so, in-presence 
>>> of
>>> >> huge L3-cache (few MBs). When the VCPU starts running some of 
>>> the
>>> >> contents guest RAM will be still in L3-cache and VCPU runs with
>>> >> MMU off (i.e. cacheing off) hence VCPU will bypass L3-cache and
>>> >> see incorrect contents. To solve this problem we need to flush 
>>> the
>>> >> guest RAM contents before they are accessed by first time by 
>>> VCPU.
>>> >>
>>> > ok, I'm with you that far.
>>> >
>>> > But is it also not true that we need to decide between:
>>> >
>>> >   A.1: Flush the entire guest RAM before running the VCPU
>>> >   A.2: Flush the pages as we fault them in
>>>
>>> Yes, thats the decision we have to make.
>>>
>>> >
>>> > And (independently):
>>> >
>>> >   B.1: Use __flush_dcache_range
>>> >   B.2: Use something else + outer cache framework for arm64
>>>
>>> This would be __flush_dcache_all() + outer cache flush all.
>>
>> We need to be careful here since the __flush_dcache_all() operation 
>> uses
>> cache maintenance by set/way and these are *local* to a CPU (IOW not
>> broadcast). Do you have any guarantee that dirty cache lines don't
>> migrate between CPUs and __flush_dcache_all() wouldn't miss them?
>> Architecturally we don't, so this is not a safe operation that would
>> guarantee L1 cache flushing (we probably need to revisit some of the
>> __flush_dcache_all() calls in KVM, I haven't looked into this).
>>
>> So I think we are left to the range operation where the DC ops to 
>> PoC
>> would be enough for your L3.
>
> If __flush_dcache_all() is *local" to a CPU then I guess DC ops to 
> PoC
> by range would be the only option.
>
>>
>> An outer cache flush all is probably only needed for cpuidle/suspend
>> (the booting part should be handled by the boot loader).
>
> Yes, cpuidle/suspend would definitely require outer cache 
> maintenance.
>
> For KVM, we can avoid flushing d-cache to PoC every time in
> coherent_icache_guest_page() by only doing it when Guest MMU is
> turned-off. This may reduce the performance penalty.

What about the I and C bits in SCTLR_EL1? Does L3 also honour these 
bits?

         M.
-- 
Fast, cheap, reliable. Pick two.