On 11/03/17 16:51 +0800, Haozhong Zhang wrote:
> 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@xxxxxxxxx>
> > > Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@xxxxxxxxx>
> > > Reported-by: Kumar, Karthik <karthik.kumar@xxxxxxxxx>
> > > ---
> > > 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;
> }
In addition, I think it's better to split this function into
pat_pfn_is_uc() and pat_pfn_is_uc_minus() to avoid additional
confusion.
Haozhong
>
> 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
> >
>
>
>
