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

[Haozhong Zhang]

On 11/03/17 17:10 +0800, Xiao Guangrong wrote:
> 
> 
> 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.
> 

I've done the test and it can compile.

arch/x86/mm/Makefile shows pat.c is compiled regardless of CONFIG_X86_PAT,
and pat_pfn_is_uc() is defined out of  #ifdef CONFIG_X86_PAT ... #endif.

Haozhong