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    SubjectRe: [PATCH v4 3/3] KVM: MMU: consider host cache mode in MMIO page check
    On 11/03/17 14:54 +0800, Xiao Guangrong wrote:
    >
    >
    > On 11/03/2017 01:53 PM, Haozhong Zhang wrote:
    > > Some reserved pages, such as those from NVDIMM DAX devices, are
    > > not for MMIO, and can be mapped with cached memory type for better
    > > performance. However, the above check misconceives those pages as
    > > MMIO. Because KVM maps MMIO pages with UC memory type, the
    > > performance of guest accesses to those pages would be harmed.
    > > Therefore, we check the host memory type by lookup_memtype() in
    > > addition and only treat UC/UC- pages as MMIO.
    > >
    > > Signed-off-by: Haozhong Zhang <haozhong.zhang@intel.com>
    > > Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@intel.com>
    > > Reported-by: Kumar, Karthik <karthik.kumar@intel.com>
    > > ---
    > > arch/x86/kvm/mmu.c | 19 ++++++++++++++++++-
    > > 1 file changed, 18 insertions(+), 1 deletion(-)
    > >
    > > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
    > > index 0b481cc9c725..e9ed0e666a83 100644
    > > --- a/arch/x86/kvm/mmu.c
    > > +++ b/arch/x86/kvm/mmu.c
    > > @@ -2708,7 +2708,24 @@ static bool mmu_need_write_protect(struct kvm_vcpu *vcpu, gfn_t gfn,
    > > static bool kvm_is_mmio_pfn(kvm_pfn_t pfn)
    > > {
    > > if (pfn_valid(pfn))
    > > - return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
    > > + return !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn)) &&
    > > + /*
    > > + * Some reserved pages, such as those from
    > > + * NVDIMM DAX devices, are not for MMIO, and
    > > + * can be mapped with cached memory type for
    > > + * better performance. However, the above
    > > + * check misconceives those pages as MMIO.
    > > + * Because KVM maps MMIO pages with UC memory
    > > + * type, the performance of guest accesses to
    > > + * those pages would be harmed. Therefore, we
    > > + * check the host memory type in addition and
    > > + * only treat UC/UC- pages as MMIO.
    > > + *
    > > + * pat_pfn_is_uc() works only when PAT is enabled,
    > > + * so check pat_enabled() as well.
    > > + */
    > > + (!pat_enabled() ||
    > > + pat_pfn_is_uc(kvm_pfn_t_to_pfn_t(pfn)));
    >
    > Can it be compiled if !CONFIG_PAT?

    Yes.

    What I check via pat_enabled() is not only whether PAT support is
    compiled, but also whether PAT is enabled at runtime.

    >
    > It would be better if we move pat_enabled out of kvm as well,

    Surely I can combine them in one function like

    bool pat_pfn_is_uc(pfn_t pfn)
    {
    enum page_cache_mode cm;

    if (!pat_enabled())
    return false;

    cm = lookup_memtype(pfn_t_to_phys(pfn));

    return cm == _PAGE_CACHE_MODE_UC || cm == _PAGE_CACHE_MODE_UC_MINUS;
    }

    but I need a good name to make its semantics clear, or is it enough to
    just leave a comment like?

    /*
    * Check via PAT whether the cache mode of a page if UC or UC-.
    *
    * Returns true, if PAT is enabled and the cache mode is UC or UC-.
    * Returns false otherwise.
    */


    > please refer
    > to pgprot_writecombine() which is implemented in pat.c and in
    > include\asm-generic\pgtable.h:
    >
    > #ifndef pgprot_writecombine
    > #define pgprot_writecombine pgprot_noncached
    > #endif
    >



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