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

[Anup Patel]

On Thu, Aug 15, 2013 at 2:01 PM, Marc Zyngier <marc.zyngier at arm.com> wrote:
> On 2013-08-15 07:26, Anup Patel wrote:
>>
>> Hi Marc,
>>
>> On Thu, Aug 15, 2013 at 10:22 AM, Marc Zyngier <marc.zyngier at arm.com>
>> wrote:
>>>
>>> Hi Anup,
>>>
>>>
>>> On 2013-08-14 15:22, Anup Patel wrote:
>>>>
>>>>
>>>> On Wed, Aug 14, 2013 at 5:34 PM, Marc Zyngier <marc.zyngier at arm.com>
>>>> 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 at linaro.org>
>>>>>> Signed-off-by: Anup Patel <anup.patel at linaro.org>
>>>>>> ---
>>>>>>  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.
>>>>
>>>>>
>>>>> 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.
>>>
>>>
>>>
>>> Can you please give the attached patch a spin on your HW? I've
>>> boot-tested
>>> it on a model, but of course I can't really verify that it fixes your
>>> issue.
>>>
>>> As far as I can see, it should fix it without any additional flushing.
>>>
>>> Please let me know how it goes.
>>
>>
>> HCR_EL2.DC=1 means all memory access for Stage1 MMU off are
>> treated as "Normal Non-shareable, Inner Write-Back Write-Allocate,
>> Outer Write-Back Write-Allocate memory"
>>
>> HCR_EL2.DC=0 means all memory access for Stage1 MMU off are
>> treated as "Strongly-ordered device memory"
>>
>> Now if Guest/VM access hardware MMIO devices directly (such as
>> VGIC CPU interface) with MMU off then MMIO devices will be
>> accessed as normal memory when HCR_EL2.DC=1.
>
>
> I don't think so. Stage-2 still applies, and should force MMIO to be
> accessed as device memory.
>
>
>> The HCR_EL2.DC=1 makes sense only if we have all software
>> emulated devices for Guest/VM which is not true for KVM ARM or
>> KVM ARM64 because we use VGIC.
>>
>> IMHO, this patch enforces incorrect memory attribute for Guest/VM
>> when Stage1 MMU is off.
>
>
> See above. My understanding is that HCR.DC controls the default output of
> Stage-1, and Stage-2 overrides still apply.

You had mentioned that PAGE_S2_DEVICE attribute was redundant
and wanted guest to decide the memory attribute. In other words, you
did not want to enforce any memory attribute in Stage2.

Please refer to this patch https://patchwork.kernel.org/patch/2543201/

>
>
>> Nevertheless, we will still try your patch.
>
>
> Thanks.
>
>
>         M.
> --
> Fast, cheap, reliable. Pick two.

Regards,
Anup