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;
}
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
>
