On 10/27/17 10:40 +0200, Ingo Molnar wrote:
>
> * Haozhong Zhang <haozhong.zh...@intel.com> wrote:
>
> > By default, KVM treats a reserved page as for MMIO purpose, and maps
> > it to guest with UC memory type. However, some reserved pages are not
> > for MMIO, such as pages of DAX device (e.g., /dev/daxX.Y). Mapping
> > them with UC memory type will harm the performance. In order to
> > exclude those cases, we check the host cache mode in addition and only
> > treat UC/UC- pages as MMIO.
> >
> > Signed-off-by: Haozhong Zhang <haozhong.zh...@intel.com>
> > Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escar...@intel.com>
> > Reported-by: Kumar, Karthik <karthik.ku...@intel.com>
> > ---
> > arch/x86/kvm/mmu.c | 32 +++++++++++++++++++++++++++++---
> > 1 file changed, 29 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index 0b481cc9c725..d4c821a6df3d 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -2707,10 +2707,36 @@ 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));
> > + bool is_mmio = true;
> >
> > - return true;
> > + if (pfn_valid(pfn)) {
> > + is_mmio = !is_zero_pfn(pfn) && PageReserved(pfn_to_page(pfn));
> > +
> > + /*
> > + * By default, KVM treats a reserved page as for MMIO
> > + * purpose, and maps it to guest with UC memory type.
> > + * However, some reserved pages are not for MMIO, such
> > + * as pages of DAX device (e.g., /dev/daxX.Y). Mapping
> > + * them with UC memory type will harm the performance.
> > + * In order to exclude those cases, we check the host
> > + * cache mode in addition and only treat UC/UC- pages
> > + * as MMIO.
> > + *
> > + * track_pfn_insert() works only when PAT is enabled,
> > + * so add pat_enabled() here.
> > + */
> > + if (is_mmio && pat_enabled()) {
> > + pgprot_t prot;
> > + enum page_cache_mode cm;
> > +
> > + track_pfn_insert(NULL, &prot, kvm_pfn_to_pfn(pfn));
> > + cm = pgprot2cachemode(prot);
> > + is_mmio = (cm == _PAGE_CACHE_MODE_UC ||
> > + cm == _PAGE_CACHE_MODE_UC_MINUS);
> > + }
> > + }
> > +
> > + return is_mmio;
> > }
> >
> > static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
>
> s/harm the performance
> /harm performance>
>
> But I suspect the rest of the comment should be rewritten too to be more
> fluid.
I'll refactor the comment.
>
> Beyond that - I think instead of exposing these low level details a properly
> named
> helper function should be put into pat.c instead - and KVM can use that.
>
KVM only needs the memory type information, so lookup_memtype() in
pat.c is probably a better one to be exposed.
Haozhong
