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

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Date:   Wed, 8 Nov 2017 11:34:40 +0800
From:   Haozhong Zhang <haozhong.zhang@...el.com>
To:     Xiao Guangrong <guangrong.xiao@...il.com>, kvm@...r.kernel.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Paolo Bonzini <pbonzini@...hat.com>, rkrcmar@...hat.com,
        Xiao Guangrong <xiaoguangrong.eric@...il.com>,
        Dan Williams <dan.j.williams@...el.com>,
        ivan.d.cuevas.escareno@...el.com, karthik.kumar@...el.com,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Olif Chapman <olif.chapman@...cle.com>,
        Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH v4 3/3] KVM: MMU: consider host cache mode in MMIO page
 check

On 11/03/17 16:51 +0800, 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@...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 | 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;

I made a mistake: it should return true here. Then the semantics of
this function is confused. I think it's still better to leave
!pat_enabled() check in KVM.

Haozhong

> 
> 	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
> >
> 
> 
>