Re: [RFC PATCH 1/4] KVM: arm64: Move the clean of dcache to the map handler

[Marc Zyngier]

On Wed, 24 Feb 2021 17:21:22 +0000,
Alexandru Elisei <alexandru.elisei@xxxxxxx> wrote:
> 
> Hello,
> 
> On 2/8/21 11:22 AM, Yanan Wang wrote:
> > We currently uniformly clean dcache in user_mem_abort() before calling the
> > fault handlers, if we take a translation fault and the pfn is cacheable.
> > But if there are concurrent translation faults on the same page or block,
> > clean of dcache for the first time is necessary while the others are not.
> >
> > By moving clean of dcache to the map handler, we can easily identify the
> > conditions where CMOs are really needed and avoid the unnecessary ones.
> > As it's a time consuming process to perform CMOs especially when flushing
> > a block range, so this solution reduces much load of kvm and improve the
> > efficiency of creating mappings.
> >
> > Signed-off-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
> > ---
> >  arch/arm64/include/asm/kvm_mmu.h | 16 --------------
> >  arch/arm64/kvm/hyp/pgtable.c     | 38 ++++++++++++++++++++------------
> >  arch/arm64/kvm/mmu.c             | 14 +++---------
> >  3 files changed, 27 insertions(+), 41 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> > index e52d82aeadca..4ec9879e82ed 100644
> > --- a/arch/arm64/include/asm/kvm_mmu.h
> > +++ b/arch/arm64/include/asm/kvm_mmu.h
> > @@ -204,22 +204,6 @@ static inline bool vcpu_has_cache_enabled(struct kvm_vcpu *vcpu)
> >  	return (vcpu_read_sys_reg(vcpu, SCTLR_EL1) & 0b101) == 0b101;
> >  }
> >  
> > -static inline void __clean_dcache_guest_page(kvm_pfn_t pfn, unsigned long size)
> > -{
> > -	void *va = page_address(pfn_to_page(pfn));
> > -
> > -	/*
> > -	 * With FWB, we ensure that the guest always accesses memory using
> > -	 * cacheable attributes, and we don't have to clean to PoC when
> > -	 * faulting in pages. Furthermore, FWB implies IDC, so cleaning to
> > -	 * PoU is not required either in this case.
> > -	 */
> > -	if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))
> > -		return;
> > -
> > -	kvm_flush_dcache_to_poc(va, size);
> > -}
> > -
> >  static inline void __invalidate_icache_guest_page(kvm_pfn_t pfn,
> >  						  unsigned long size)
> >  {
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 4d177ce1d536..2f4f87021980 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -464,6 +464,26 @@ static int stage2_map_set_prot_attr(enum kvm_pgtable_prot prot,
> >  	return 0;
> >  }
> >  
> > +static bool stage2_pte_cacheable(kvm_pte_t pte)
> > +{
> > +	u64 memattr = pte & KVM_PTE_LEAF_ATTR_LO_S2_MEMATTR;
> > +	return memattr == PAGE_S2_MEMATTR(NORMAL);
> > +}
> > +
> > +static void stage2_flush_dcache(void *addr, u64 size)
> > +{
> > +	/*
> > +	 * With FWB, we ensure that the guest always accesses memory using
> > +	 * cacheable attributes, and we don't have to clean to PoC when
> > +	 * faulting in pages. Furthermore, FWB implies IDC, so cleaning to
> > +	 * PoU is not required either in this case.
> > +	 */
> > +	if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))
> > +		return;
> > +
> > +	__flush_dcache_area(addr, size);
> > +}
> > +
> >  static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> >  				      kvm_pte_t *ptep,
> >  				      struct stage2_map_data *data)
> > @@ -495,6 +515,10 @@ static int stage2_map_walker_try_leaf(u64 addr, u64 end, u32 level,
> >  		put_page(page);
> >  	}
> >  
> > +	/* Flush data cache before installation of the new PTE */
> > +	if (stage2_pte_cacheable(new))
> > +		stage2_flush_dcache(__va(phys), granule);
> 
> This makes sense to me. kvm_pgtable_stage2_map() is protected
> against concurrent calls by the kvm->mmu_lock, so only one VCPU can
> change the stage 2 translation table at any given moment. In the
> case of concurrent translation faults on the same IPA, the first
> VCPU that will take the lock will create the mapping and do the
> dcache clean+invalidate. The other VCPUs will return -EAGAIN because
> the mapping they are trying to install is almost identical* to the
> mapping created by the first VCPU that took the lock.
> 
> I have a question. Why are you doing the cache maintenance *before*
> installing the new mapping? This is what the kernel already does, so
> I'm not saying it's incorrect, I'm just curious about the reason
> behind it.

The guarantee KVM offers to the guest is that by the time it can
access the memory, it is cleaned to the PoC. If you establish a
mapping before cleaning, another vcpu can access the PoC (no fault,
you just set up S2) and not see it up to date.

Thanks,

	M.

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