Re: [PATCH v4 3/3] KVM: MMU: consider host cache mode in MMIO page check

[Xiao Guangrong]

On 11/03/2017 04:51 PM, 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.

The issue is about pat_pfn_is_uc() which is implemented only if CONFIG_PAT is
enabled, but you used it here unconditionally.

I am not sure if gcc is smart enough to omit pat_pfn_is_uc() completely under
this case. If you really have done the test to compile kernel and KVM module
with CONFIG_PAT disabled, it is fine.