Subject: Re: [PATCH v10 2/3] arm64: mm: implement arch_faults_on_old_pte() on arm64

From: Marc Zyngier <[email-masked]>
Date: Tue, 1 Oct 2019 14:32:19 +0100

On Tue, 1 Oct 2019 13:50:32 +0100
Will Deacon <[email-masked]> wrote:

> On Mon, Sep 30, 2019 at 09:57:39AM +0800, Jia He wrote:
> > On arm64 without hardware Access Flag, copying fromuser will fail because
> > the pte is old and cannot be marked young. So we always end up with zeroed
> > page after fork() + CoW for pfn mappings. we don't always have a
> > hardware-managed access flag on arm64.
> > 
> > Hence implement arch_faults_on_old_pte on arm64 to indicate that it might
> > cause page fault when accessing old pte.
> > 
> > Signed-off-by: Jia He <[email-masked]>
> > Reviewed-by: Catalin Marinas <[email-masked]>
> > ---
> >  arch/arm64/include/asm/pgtable.h | 14 ++++++++++++++
> >  1 file changed, 14 insertions(+)
> > 
> > diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h
> > index 7576df00eb50..e96fb82f62de 100644
> > --- a/arch/arm64/include/asm/pgtable.h
> > +++ b/arch/arm64/include/asm/pgtable.h
> > @@ -885,6 +885,20 @@ static inline void update_mmu_cache(struct vm_area_struct *vma,
> >  #define phys_to_ttbr(addr)	(addr)
> >  #endif
> >  
> > +/*
> > + * On arm64 without hardware Access Flag, copying from user will fail because
> > + * the pte is old and cannot be marked young. So we always end up with zeroed
> > + * page after fork() + CoW for pfn mappings. We don't always have a
> > + * hardware-managed access flag on arm64.
> > + */
> > +static inline bool arch_faults_on_old_pte(void)
> > +{
> > +	WARN_ON(preemptible());
> > +
> > +	return !cpu_has_hw_af();
> > +}  
> 
> Does this work correctly in a KVM guest? (i.e. is the MMFR sanitised in that
> case, despite not being the case on the host?)

Yup, all the 64bit MMFRs are trapped (HCR_EL2.TID3 is set for an
AArch64 guest), and we return the sanitised version.

But that's an interesting remark: we're now trading an extra fault on
CPUs that do not support HWAFDBS for a guaranteed trap for each and
every guest under the sun that will hit the COW path...

My gut feeling is that this is going to be pretty visible. Jia, do you
have any numbers for this kind of behaviour?

Thanks,

	M.
-- 
Without deviation from the norm, progress is not possible.

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