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Date: Thu, 8 Feb 2024 13:24:59 +0000
From: Oliver Upton <oliver.upton@linux.dev>
To: Catalin Marinas <catalin.marinas@arm.com>
Cc: ankita@nvidia.com, jgg@nvidia.com, maz@kernel.org, james.morse@arm.com,
	suzuki.poulose@arm.com, yuzenghui@huawei.com,
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Subject: Re: [PATCH v6 1/4] kvm: arm64: introduce new flag for non-cacheable
 IO memory
Message-ID: <ZcTWK6TksvugSlI-@linux.dev>
References: <20240207204652.22954-1-ankita@nvidia.com>
 <20240207204652.22954-2-ankita@nvidia.com>
 <ZcTQi0wWZgvl05LB@arm.com>
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In-Reply-To: <ZcTQi0wWZgvl05LB@arm.com>

On Thu, Feb 08, 2024 at 01:00:59PM +0000, Catalin Marinas wrote:
> On Thu, Feb 08, 2024 at 02:16:49AM +0530, ankita@nvidia.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