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    SubjectRe: [PATCH 4/7] KVM: MMU: don't mark pte notrap if it's just sync transient


    Marcelo Tosatti wrote:
    > On Fri, Jun 11, 2010 at 09:31:38PM +0800, Xiao Guangrong wrote:
    >> If the sync-sp just sync transient, don't mark its pte notrap
    >>
    >> Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
    >> ---
    >> arch/x86/include/asm/kvm_host.h | 2 +-
    >> arch/x86/kvm/mmu.c | 11 ++++-------
    >> arch/x86/kvm/paging_tmpl.h | 5 +++--
    >> 3 files changed, 8 insertions(+), 10 deletions(-)
    >
    > Xiao,
    >
    > Can you explain the reasoning for this?
    >

    Marcelo,

    In the kvm_sync_page_transient() path, the sp is keep unsync and sp->gfn is not
    write protected, we can't set 'spte == shadow_notrap_nonpresent_pte' if the
    'gpte.p == 0' in this case.

    It's because if 'gpte.p == 0', when guest change the gpte's mapping, it's no need
    to flush tlb(p == 0 means the mapping is not in tlb), if we set spte to
    'shadow_notrap_nonpresent_pte', we will miss the chance to update the mapping, also
    a incorrect #PF will pass to guest directly, may cause guest crash.

    And, this is the reasoning we do mmu_convert_notrap() in kvm_sync_page_transient()
    before, this patch just avoid this unnecessary workload. :-)



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