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

[Anup Patel]

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.

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