On 14.08.2013, at 16:22, Anup Patel wrote:
> On Wed, Aug 14, 2013 at 5:34 PM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>> Hi Pranav,
>>
>> On 2013-08-14 12:47, Pranavkumar Sawargaonkar wrote:
>>> Systems with large external L3-cache (few MBs), might have dirty
>>> content belonging to the guest page in L3-cache. To tackle this,
>>> we need to flush such dirty content from d-cache so that guest
>>> will see correct contents of guest page when guest MMU is disabled.
>>>
>>> The patch fixes coherent_icache_guest_page() for external L3-cache.
>>>
>>> Signed-off-by: Pranavkumar Sawargaonkar <pranavkumar@xxxxxxxxxx>
>>> Signed-off-by: Anup Patel <anup.patel@xxxxxxxxxx>
>>> ---
>>> arch/arm64/include/asm/kvm_mmu.h | 2 ++
>>> 1 file changed, 2 insertions(+)
>>>
>>> diff --git a/arch/arm64/include/asm/kvm_mmu.h
>>> b/arch/arm64/include/asm/kvm_mmu.h
>>> index efe609c..5129038 100644
>>> --- a/arch/arm64/include/asm/kvm_mmu.h
>>> +++ b/arch/arm64/include/asm/kvm_mmu.h
>>> @@ -123,6 +123,8 @@ static inline void
>>> coherent_icache_guest_page(struct kvm *kvm, gfn_t gfn)
>>> if (!icache_is_aliasing()) { /* PIPT */
>>> unsigned long hva = gfn_to_hva(kvm, gfn);
>>> flush_icache_range(hva, hva + PAGE_SIZE);
>>> + /* Flush d-cache for systems with external caches. */
>>> + __flush_dcache_area((void *) hva, PAGE_SIZE);
>>> } else if (!icache_is_aivivt()) { /* non ASID-tagged VIVT */
>>> /* any kind of VIPT cache */
>>> __flush_icache_all();
>>
>> [adding Will to the discussion as we talked about this in the past]
>>
>> That's definitely an issue, but I'm not sure the fix is to hit the data
>> cache on each page mapping. It looks overkill.
>>
>> Wouldn't it be enough to let userspace do the cache cleaning? kvmtools
>> knows which bits of the guest memory have been touched, and can do a "DC
>> DVAC" on this region.
>
> It seems a bit unnatural to have cache cleaning is user-space. I am sure
> other architectures don't have such cache cleaning in user-space for KVM.
Not sure I understand at which point you really need to make things coherent here. When you assign a new ASID because there could be stale cache entries left from an old ASID run? Can't you just flush all caches when you overrun your ASID allocator?
>
>>
>> The alternative is do it in the kernel before running any vcpu - but
>> that's not very nice either (have to clean the whole of the guest
>> memory, which makes a full dcache clean more appealing).
>
> Actually, cleaning full d-cache by set/way upon first run of VCPU was
> our second option but current approach seemed very simple hence
> we went for this.
>
> If more people vote for full d-cache clean upon first run of VCPU then
> we should revise this patch.
Is there any difference in performance between the 2 approaches?
Alex
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