• Catalin Marinas's avatar
    kvm: arm64: Enable hardware updates of the Access Flag for Stage 2 page tables · 06485053
    Catalin Marinas authored
    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: default avatarCatalin Marinas <catalin.marinas@arm.com>
    Cc: Paolo Bonzini <pbonzini@redhat.com>
    Acked-by: default avatarMarc Zyngier <marc.zyngier@arm.com>
    Reviewed-by: default avatarChristoffer Dall <christoffer.dall@linaro.org>
    Signed-off-by: default avatarChristoffer Dall <christoffer.dall@linaro.org>
    06485053
pgtable.h 20.3 KB