linux-kernel - Re: [PATCH v6 1/4] kvm: arm64: introduce new flag for non-cacheable IO memory

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Date: Thu, 8 Feb 2024 13:24:59 +0000
From: Oliver Upton <oliver.upton@...ux.dev>
To: Catalin Marinas <catalin.marinas@....com>
Cc: ankita@...dia.com, jgg@...dia.com, maz@...nel.org, james.morse@....com,
	suzuki.poulose@....com, yuzenghui@...wei.com,
	reinette.chatre@...el.com, surenb@...gle.com, stefanha@...hat.com,
	brauner@...nel.org, will@...nel.org, mark.rutland@....com,
	alex.williamson@...hat.com, kevin.tian@...el.com,
	yi.l.liu@...el.com, ardb@...nel.org, akpm@...ux-foundation.org,
	andreyknvl@...il.com, wangjinchao@...sion.com, gshan@...hat.com,
	ricarkol@...gle.com, linux-mm@...ck.org, lpieralisi@...nel.org,
	rananta@...gle.com, ryan.roberts@....com, aniketa@...dia.com,
	cjia@...dia.com, kwankhede@...dia.com, targupta@...dia.com,
	vsethi@...dia.com, acurrid@...dia.com, apopple@...dia.com,
	jhubbard@...dia.com, danw@...dia.com, kvmarm@...ts.linux.dev,
	mochs@...dia.com, zhiw@...dia.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v6 1/4] kvm: arm64: introduce new flag for non-cacheable
 IO memory

On Thu, Feb 08, 2024 at 01:00:59PM +0000, Catalin Marinas wrote:
> On Thu, Feb 08, 2024 at 02:16:49AM +0530, ankita@...dia.com wrote:
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index c651df904fe3..2a893724ee9b 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -717,15 +717,28 @@ void kvm_tlb_flush_vmid_range(struct kvm_s2_mmu *mmu,
> >  static int stage2_set_prot_attr(struct kvm_pgtable *pgt, enum kvm_pgtable_prot prot,
> >  				kvm_pte_t *ptep)
> >  {
> > -	bool device = prot & KVM_PGTABLE_PROT_DEVICE;
> > -	kvm_pte_t attr = device ? KVM_S2_MEMATTR(pgt, DEVICE_nGnRE) :
> > -			    KVM_S2_MEMATTR(pgt, NORMAL);
> > +	kvm_pte_t attr;
> >  	u32 sh = KVM_PTE_LEAF_ATTR_LO_S2_SH_IS;
> >  
> > +	switch (prot & (KVM_PGTABLE_PROT_DEVICE |
> > +			KVM_PGTABLE_PROT_NORMAL_NC)) {
> > +	case 0:
> > +		attr = KVM_S2_MEMATTR(pgt, NORMAL);
> > +		break;
> > +	case KVM_PGTABLE_PROT_DEVICE:
> > +		if (prot & KVM_PGTABLE_PROT_X)
> > +			return -EINVAL;
> > +		attr = KVM_S2_MEMATTR(pgt, DEVICE_nGnRE);
> > +		break;
> > +	case KVM_PGTABLE_PROT_NORMAL_NC:
> > +		attr = KVM_S2_MEMATTR(pgt, NORMAL_NC);
> > +		break;
> 
> Does it make sense to allow executable here as well? I don't think it's
> harmful but not sure there's a use-case for it either.

Ah, we should just return EINVAL for that too.

I get that the memory attribute itself is not problematic, but since
we're only using this thing for MMIO it'd be a rather massive
bug in KVM... We reject attempts to do this earlier in user_mem_abort().

If, for some reason, we wanted to do Normal-NC actual memory then we
would need to make sure that KVM does the appropriate cache maintenance
at map / unmap.

-- 
Thanks,
Oliver