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

[Haozhong Zhang]

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
>