On 11/01/15 12:30, Christoffer Dall wrote:
> On Fri, Jan 09, 2015 at 02:35:49PM +0000, Marc Zyngier wrote:
>> On 09/01/15 12:30, Christoffer Dall wrote:
>>> On Thu, Jan 08, 2015 at 11:59:08AM +0000, Marc Zyngier wrote:
>>>> Let's assume a guest has created an uncached mapping, and written
>>>> to that page. Let's also assume that the host uses a cache-coherent
>>>> IO subsystem. Let's finally assume that the host is under memory
>>>> pressure and starts to swap things out.
>>>>
>>>> Before this "uncached" page is evicted, we need to make sure it
>>>> gets invalidated, or the IO subsystem is going to swap out the
>>>> cached view, loosing the data that has been written there.
>>>>
>>>> Signed-off-by: Marc Zyngier <marc.zyngier@xxxxxxx>
>>>> ---
>>>> arch/arm/include/asm/kvm_mmu.h | 31 +++++++++++++++++++++++++++
>>>> arch/arm/kvm/mmu.c | 46 +++++++++++++++++++++++++++-------------
>>>> arch/arm64/include/asm/kvm_mmu.h | 18 ++++++++++++++++
>>>> 3 files changed, 80 insertions(+), 15 deletions(-)
>>>>
>>>> diff --git a/arch/arm/include/asm/kvm_mmu.h b/arch/arm/include/asm/kvm_mmu.h
>>>> index 63e0ecc..7ceb836 100644
>>>> --- a/arch/arm/include/asm/kvm_mmu.h
>>>> +++ b/arch/arm/include/asm/kvm_mmu.h
>>>> @@ -44,6 +44,7 @@
>>>>
>>>> #ifndef __ASSEMBLY__
>>>>
>>>> +#include <linux/highmem.h>
>>>> #include <asm/cacheflush.h>
>>>> #include <asm/pgalloc.h>
>>>>
>>>> @@ -190,6 +191,36 @@ static inline void coherent_cache_guest_page(struct kvm_vcpu *vcpu, hva_t hva,
>>>>
>>>> #define kvm_virt_to_phys(x) virt_to_idmap((unsigned long)(x))
>>>>
>>>> +static inline void __kvm_flush_dcache_pte(pte_t pte)
>>>> +{
>>>> + void *va = kmap_atomic(pte_page(pte));
>>>> +
>>>> + kvm_flush_dcache_to_poc(va, PAGE_SIZE);
>>>> +
>>>> + kunmap_atomic(va);
>>>> +}
>>>> +
>>>> +static inline void __kvm_flush_dcache_pmd(pmd_t pmd)
>>>> +{
>>>> + unsigned long size = PMD_SIZE;
>>>> + pfn_t pfn = pmd_pfn(pmd);
>>>> +
>>>> + while (size) {
>>>> + void *va = kmap_atomic_pfn(pfn);
>>>> +
>>>> + kvm_flush_dcache_to_poc(va, PAGE_SIZE);
>>>> +
>>>> + pfn++;
>>>> + size -= PAGE_SIZE;
>>>> +
>>>> + kunmap_atomic(va);
>>>> + }
>>>> +}
>>>> +
>>>> +static inline void __kvm_flush_dcache_pud(pud_t pud)
>>>> +{
>>>> +}
>>>> +
>>>> void stage2_flush_vm(struct kvm *kvm);
>>>>
>>>> #endif /* !__ASSEMBLY__ */
>>>> diff --git a/arch/arm/kvm/mmu.c b/arch/arm/kvm/mmu.c
>>>> index 1dc9778..1f5b793 100644
>>>> --- a/arch/arm/kvm/mmu.c
>>>> +++ b/arch/arm/kvm/mmu.c
>>>> @@ -58,6 +58,21 @@ static void kvm_tlb_flush_vmid_ipa(struct kvm *kvm, phys_addr_t ipa)
>>>> kvm_call_hyp(__kvm_tlb_flush_vmid_ipa, kvm, ipa);
>>>> }
>>>>
>>>> +static void kvm_flush_dcache_pte(pte_t pte)
>>>> +{
>>>> + __kvm_flush_dcache_pte(pte);
>>>> +}
>>>> +
>>>> +static void kvm_flush_dcache_pmd(pmd_t pmd)
>>>> +{
>>>> + __kvm_flush_dcache_pmd(pmd);
>>>> +}
>>>> +
>>>> +static void kvm_flush_dcache_pud(pud_t pud)
>>>> +{
>>>> + __kvm_flush_dcache_pud(pud);
>>>> +}
>>>> +
>>>> static int mmu_topup_memory_cache(struct kvm_mmu_memory_cache *cache,
>>>> int min, int max)
>>>> {
>>>> @@ -128,9 +143,12 @@ static void unmap_ptes(struct kvm *kvm, pmd_t *pmd,
>>>> start_pte = pte = pte_offset_kernel(pmd, addr);
>>>> do {
>>>> if (!pte_none(*pte)) {
>>>> + pte_t old_pte = *pte;
>>>> kvm_set_pte(pte, __pte(0));
>>>> - put_page(virt_to_page(pte));
>>>
>>> was this a bug beforehand that we released the page before we flushed
>>> the TLB?
>>
>> I don't think so. TLB maintenance doesn't require the mapping to exist
>> in the page tables (while the cache maintenance do).
>>
>
> duh, the put_page is the ref counting on the page table itself, not the
> underlying memory page. Forget what I asked.
>
>>>> kvm_tlb_flush_vmid_ipa(kvm, addr);
>>>> + if ((pte_val(old_pte) & PAGE_S2_DEVICE) != PAGE_S2_DEVICE)
>>>> + kvm_flush_dcache_pte(old_pte);
>>>
>>> this is confusing me: We are only flushing the cache for cached stage-2
>>> mappings? Weren't we trying to flush the cache for uncached mappings?
>>> (we obviously also need flush a cached stage-2 mapping but where the
>>> guest is mapping it as uncached, but we don't know that easily).
>>>
>>> Am I missing something completely here?
>>
>> I think you must be misreading something:
>> - we want to invalidate mappings because the guest may have created an
>> uncached mapping
>
> I don't quite understand: we are invalidating mappings because the page
> is being swapped out (and the guest must fault if it tries to access it
> again). Not because the guest may have created an uncached mapping,
> that's just an aspect of the situation. Or am I thinking about this the
> wrong way?
My wording was quite ambiguous. Indeed, we're removing the mapping
because the page is being evicted. In a perfect and ideal world (where
the guest doesn't do anything silly and everything is cache coherent),
we shouldn't have to do anything cache wise. But see below.
>> - as we cannot know about the guest's uncached mappings, we flush things
>> unconditionally (basically anything that is RAM).
>
> so isn't the problem that the host may have an invalid cached view of
> the data, so we need to invalidate that view, not flush the invalid data
> to RAM? Does the kernel take care of that somewhere else for a
> cache-coherent IO subsystem?
There is two potential problems:
- The guest has created an uncached mapping, and you have a cache
coherent IO subsystem: we need to invalidate the cached view. But since
we don't know where that mapping is (as we don't track guest mappings),
we must do a clean+invalidate in order not to corrupt cached mappings.
- The guest has cached mappings (that's the usual case), and the IO
subsystem is not cache-coherent. In this case, the kernel knows about
this and will do the clean operation for us.
The main problem is that we cannot identify any of these two cases: we
don't know if the IO path is coherent or not, and we don't know about
uncached mappings. The only safe thing to do is to perform the
clean+invalidate, always.
I wish we had a way to identify uncached mappings, That would save us a
lot of over-maintenance and some very tricky games we play at the MMU level.
Does the above make things clearer?
Thanks,
M.
--
Jazz is not dead. It just smells funny...
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