• Avi Kivity's avatar
    KVM: MMU: Optimize pte permission checks · 97d64b78
    Avi Kivity authored
    walk_addr_generic() permission checks are a maze of branchy code, which is
    performed four times per lookup.  It depends on the type of access, efer.nxe,
    cr0.wp, cr4.smep, and in the near future, cr4.smap.
    Optimize this away by precalculating all variants and storing them in a
    bitmap.  The bitmap is recalculated when rarely-changing variables change
    (cr0, cr4) and is indexed by the often-changing variables (page fault error
    code, pte access permissions).
    The permission check is moved to the end of the loop, otherwise an SMEP
    fault could be reported as a false positive, when PDE.U=1 but PTE.U=0.
    Noted by Xiao Guangrong.
    The result is short, branch-free code.
    Reviewed-by: default avatarXiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
    Signed-off-by: default avatarAvi Kivity <avi@redhat.com>