linux-kernel - Re: [PATCH v2 2/2] KVM: MMU: consider host cache mode in MMIO page check

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Date:   Thu, 2 Nov 2017 17:20:58 +0800
From:   Haozhong Zhang <haozhong.zhang@...el.com>
To:     Xiao Guangrong <xiaoguangrong.eric@...il.com>
Cc:     kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com,
        Dan Williams <dan.j.williams@...el.com>,
        ivan.d.cuevas.escareno@...el.com, karthik.kumar@...el.com,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v2 2/2] KVM: MMU: consider host cache mode in MMIO page
 check

On 11/02/17 15:56 +0800, Xiao Guangrong wrote:
> 
> 
> On 10/31/2017 07:48 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@...el.com>
> > Reported-by: Cuevas Escareno, Ivan D <ivan.d.cuevas.escareno@...el.com>
> > Reported-by: Kumar, Karthik <karthik.kumar@...el.com>
> > ---
> >   arch/x86/kvm/mmu.c | 30 +++++++++++++++++++++++++++---
> >   1 file changed, 27 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> > index 0b481cc9c725..206828d18857 100644
> > --- a/arch/x86/kvm/mmu.c
> > +++ b/arch/x86/kvm/mmu.c
> > @@ -2707,10 +2707,34 @@ 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));
> > +
> > +		/*
> > +		 * 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.
> > +		 *
> > +		 * lookup_memtype() works only when PAT is enabled, so
> > +		 * add pat_enabled() check here.
> > +		 */
> > +		if (is_mmio && pat_enabled()) {
> > +			enum page_cache_mode cm = lookup_memtype(PFN_PHYS(pfn));
> > +
> > +			is_mmio = (cm == _PAGE_CACHE_MODE_UC ||
> > +				   cm == _PAGE_CACHE_MODE_UC_MINUS);
> > +		}
> > +	}
> 
> You can move all of these detailed stuffs to pat.c and abstract them by introducing
> a function, maybe named pat_pfn_is_uc(). I think this is what Ingo wants.
> 

OK, I'll move the cache mode check to a function in pat.c.

Let me wait for Paolo and others' comments. If no additional comments,
I'll send another version ASAP.

Thanks,
Haozhong