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).
>> 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
- as we cannot know about the guest's uncached mappings, we flush things
unconditionally (basically anything that is RAM).
- we avoid flushing devices because it is pointless (and the kernel
doesn't have a linear mapping for those).
Is that clearer? Or is it me that is misunderstanding your concerns
(entirely possible, I'm not at my best right now...).
Thanks,
M.
--
Jazz is not dead. It just smells funny...
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