Re: Memory regions and VMAs across architectures

From: Ard Biesheuvel <Ard.Biesheuvel@arm.com>
To: Christoffer Dall <Christoffer.Dall@arm.com>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Cc: Marc Zyngier <maz@kernel.org>,
	"sean.j.christopherson@intel.com"
	<sean.j.christopherson@intel.com>,
	"borntraeger@de.ibm.com" <borntraeger@de.ibm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	"kvmarm@lists.cs.columbia.edu" <kvmarm@lists.cs.columbia.edu>,
	"linux-arm-kernel@lists.infradead.org"
	<linux-arm-kernel@lists.infradead.org>
Subject: Re: Memory regions and VMAs across architectures
Date: Fri, 8 Nov 2019 13:59:26 +0000	[thread overview]
Message-ID: <d49efeb7-3cad-9400-5e67-8a1e80ef7407@arm.com> (raw)
In-Reply-To: <20191108111920.GD17608@e113682-lin.lund.arm.com>

On 11/8/19 12:19 PM, Christoffer Dall wrote:
> Hi,
>
> I had a look at our relatively complicated logic in
> kvm_arch_prepare_memory_region(), and was wondering if there was room to
> unify some of this handling between architectures.
>
> (If you haven't seen our implementation, you can find it in
> virt/kvm/arm/mmu.c, and it has lovely ASCII art!)
>
> I then had a look at the x86 code, but that doesn't actually do anything
> when creating memory regions, which makes me wonder why the arhitectures
> differ in this aspect.
>
> The reason we added the logic that we have for arm/arm64 is that we
> don't really want to take faults for I/O accesses.  I'm not actually
> sure if this is a corretness thing, or an optimization effort, and the
> original commit message doesn't really explain.  Ard, you wrote that
> code, do you recall the details?
>

I have a vague recollection of implementing execution from read-only
guest memory in order to support execute-in-place from emulated NOR
flash in UEFI, and going down a rabbit hole debugging random, seemingly
unrelated crashes in the host which turned out to be caused by the zero
page getting corrupted because it was mapped read-write in the guest to
back uninitialized regions of the NOR flash.

That doesn't quite answer your question, though - I think it was just an
optimization ...

> In any case, what we do is to check for each VMA backing a memslot, we
> check if the memslot flags and vma flags are a reasonable match, and we
> try to detect I/O mappings by looking for the VM_PFNMAP flag on the VMA
> and pre-populate stage 2 page tables (our equivalent of EPT/NPT/...).
> However, there are some things which are not clear to me:
>
> First, what prevents user space from messing around with the VMAs after
> kvm_arch_prepare_memory_region() completes?  If nothing, then what is
> the value of the cheks we perform wrt. to VMAs?
>
> Second, why would arm/arm64 need special handling for I/O mappings
> compared to other architectures, and how is this dealt with for
> x86/s390/power/... ?
>
>
> Thanks,
>
>      Christoffer
>

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