On 13/04/16 17:57, Catalin Marinas wrote: > The ARMv8.1 architecture extensions introduce support for hardware > updates of the access and dirty information in page table entries. With > VTCR_EL2.HA enabled (bit 21), when the CPU accesses an IPA with the > PTE_AF bit cleared in the stage 2 page table, instead of raising an > Access Flag fault to EL2 the CPU sets the actual page table entry bit > (10). To ensure that kernel modifications to the page table do not > inadvertently revert a bit set by hardware updates, certain Stage 2 > software pte/pmd operations must be performed atomically. > > The main user of the AF bit is the kvm_age_hva() mechanism. The > kvm_age_hva_handler() function performs a "test and clear young" action > on the pte/pmd. This needs to be atomic in respect of automatic hardware > updates of the AF bit. Since the AF bit is in the same position for both > Stage 1 and Stage 2, the patch reuses the existing > ptep_test_and_clear_young() functionality if > __HAVE_ARCH_PTEP_TEST_AND_CLEAR_YOUNG is defined. Otherwise, the > existing pte_young/pte_mkold mechanism is preserved. > > The kvm_set_s2pte_readonly() (and the corresponding pmd equivalent) have > to perform atomic modifications in order to avoid a race with updates of > the AF bit. The arm64 implementation has been re-written using > exclusives. > > Currently, kvm_set_s2pte_writable() (and pmd equivalent) take a pointer > argument and modify the pte/pmd in place. However, these functions are > only used on local variables rather than actual page table entries, so > it makes more sense to follow the pte_mkwrite() approach for stage 1 > attributes. The change to kvm_s2pte_mkwrite() makes it clear that these > functions do not modify the actual page table entries. > > The (pte|pmd)_mkyoung() uses on Stage 2 entries (setting the AF bit > explicitly) do not need to be modified since hardware updates of the > dirty status are not supported by KVM, so there is no possibility of > losing such information. > > Signed-off-by: Catalin Marinas <catalin.marinas@xxxxxxx> > Cc: Christoffer Dall <christoffer.dall@xxxxxxxxxx> > Cc: Marc Zyngier <marc.zyngier@xxxxxxx> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> Acked-by: Marc Zyngier <marc.zyngier@xxxxxxx> Christoffer: assuming you're happy with that patch, could you queue it for 4.7? Thanks, M. -- Jazz is not dead. It just smells funny... _______________________________________________ kvmarm mailing list kvmarm@xxxxxxxxxxxxxxxxxxxxx https://lists.cs.columbia.edu/mailman/listinfo/kvmarm
