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

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, reinette.chatre@intel.com,
	surenb@google.com, stefanha@redhat.com, brauner@kernel.org,
	will@kernel.org, mark.rutland@arm.com,
	alex.williamson@redhat.com, kevin.tian@intel.com,
	yi.l.liu@intel.com, ardb@kernel.org, akpm@linux-foundation.org,
	andreyknvl@gmail.com, wangjinchao@xfusion.com, gshan@redhat.com,
	ricarkol@google.com, linux-mm@kvack.org, lpieralisi@kernel.org,
	rananta@google.com, ryan.roberts@arm.com, aniketa@nvidia.com,
	cjia@nvidia.com, kwankhede@nvidia.com, targupta@nvidia.com,
	vsethi@nvidia.com, acurrid@nvidia.com, apopple@nvidia.com,
	jhubbard@nvidia.com, danw@nvidia.com, kvmarm@lists.linux.dev,
	mochs@nvidia.com, zhiw@nvidia.com, kvm@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v6 1/4] kvm: arm64: introduce new flag for non-cacheable IO memory
Date: Thu, 8 Feb 2024 13:24:59 +0000	[thread overview]
Message-ID: <ZcTWK6TksvugSlI-@linux.dev> (raw)
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