• Paul Mackerras's avatar
    KVM: PPC: Book3S PR: Better handling of host-side read-only pages · 93b159b4
    Paul Mackerras authored
    Currently we request write access to all pages that get mapped into the
    guest, even if the guest is only loading from the page.  This reduces
    the effectiveness of KSM because it means that we unshare every page we
    access.  Also, we always set the changed (C) bit in the guest HPTE if
    it allows writing, even for a guest load.
    
    This fixes both these problems.  We pass an 'iswrite' flag to the
    mmu.xlate() functions and to kvmppc_mmu_map_page() to indicate whether
    the access is a load or a store.  The mmu.xlate() functions now only
    set C for stores.  kvmppc_gfn_to_pfn() now calls gfn_to_pfn_prot()
    instead of gfn_to_pfn() so that it can indicate whether we need write
    access to the page, and get back a 'writable' flag to indicate whether
    the page is writable or not.  If that 'writable' flag is clear, we then
    make the host HPTE read-only even if the guest HPTE allowed writing.
    
    This means that we can get a protection fault when the guest writes to a
    page that it has mapped read-write but which is read-only on the host
    side (perhaps due to KSM having merged the page).  Thus we now call
    kvmppc_handle_pagefault() for protection faults as well as HPTE not found
    faults.  In kvmppc_handle_pagefault(), if the access was allowed by the
    guest HPTE and we thus need to install a new host HPTE, we then need to
    remove the old host HPTE if there is one.  This is done with a new
    function, kvmppc_mmu_unmap_page(), which uses kvmppc_mmu_pte_vflush() to
    find and remove the old host HPTE.
    
    Since the memslot-related functions require the KVM SRCU read lock to
    be held, this adds srcu_read_lock/unlock pairs around the calls to
    kvmppc_handle_pagefault().
    
    Finally, this changes kvmppc_mmu_book3s_32_xlate_pte() to not ignore
    guest HPTEs that don't permit access, and to return -EPERM for accesses
    that are not permitted by the page protections.
    Signed-off-by: default avatarPaul Mackerras <paulus@samba.org>
    Signed-off-by: default avatarAlexander Graf <agraf@suse.de>
    93b159b4
book3s_pr.c 38.3 KB