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From: Oliver Upton To: Marc Zyngier Cc: kvmarm@lists.cs.columbia.edu, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, James Morse , Suzuki K Poulose , Alexandru Elisei , Ard Biesheuvel , Will Deacon , Quentin Perret , stable@vger.kernel.org Subject: Re: [PATCH 1/3] KVM: arm64: Fix S1PTW handling on RO memslots Message-ID: References: <20221220200923.1532710-1-maz@kernel.org> <20221220200923.1532710-2-maz@kernel.org> <86pmcdaylx.wl-maz@kernel.org> <86k02kbq2x.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <86k02kbq2x.wl-maz@kernel.org> X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Wed, Dec 21, 2022 at 05:53:58PM +0000, Marc Zyngier wrote: > On Wed, 21 Dec 2022 16:50:30 +0000, > Oliver Upton wrote: > > > > On Wed, Dec 21, 2022 at 09:35:06AM +0000, Marc Zyngier wrote: > > > > [...] > > > > > > > + if (kvm_vcpu_abt_iss1tw(vcpu)) { > > > > > + /* > > > > > + * Only a permission fault on a S1PTW should be > > > > > + * considered as a write. Otherwise, page tables baked > > > > > + * in a read-only memslot will result in an exception > > > > > + * being delivered in the guest. > > > > > > > > Somewhat of a tangent, but: > > > > > > > > Aren't we somewhat unaligned with the KVM UAPI by injecting an > > > > exception in this case? I know we've been doing it for a while, but it > > > > flies in the face of the rules outlined in the > > > > KVM_SET_USER_MEMORY_REGION documentation. > > > > > > That's an interesting point, and I certainly haven't considered that > > > for faults introduced by page table walks. > > > > > > I'm not sure what userspace can do with that though. The problem is > > > that this is a write for which we don't have useful data: although we > > > know it is a page-table walker access, we don't know what it was about > > > to write. The instruction that caused the write is meaningless (it > > > could either be a load, a store, or an instruction fetch). How do you > > > populate the data[] field then? > > > > > > If anything, this is closer to KVM_EXIT_ARM_NISV, for which we give > > > userspace the full ESR and ask it to sort it out. I doubt it will be > > > able to, but hey, maybe it is worth a shot. This would need to be a > > > different exit reason though, as NISV is explicitly for non-memslot > > > stuff. > > > > > > In any case, the documentation for KVM_SET_USER_MEMORY_REGION needs to > > > reflect the fact that KVM_EXIT_MMIO cannot represent a fault due to a > > > S1 PTW. > > > > Oh I completely agree with you here. I probably should have said before, > > I think the exit would be useless anyway. Getting the documentation in > > line with the intended behavior seems to be the best fix. > > Right. How about something like this? Looks good to me, thanks! Reviewed-by: Oliver Upton > diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst > index 226b40baffb8..72abd018a618 100644 > --- a/Documentation/virt/kvm/api.rst > +++ b/Documentation/virt/kvm/api.rst > @@ -1381,6 +1381,14 @@ It is recommended to use this API instead of the KVM_SET_MEMORY_REGION ioctl. > The KVM_SET_MEMORY_REGION does not allow fine grained control over memory > allocation and is deprecated. > > +Note: On arm64, a write generated by the page-table walker (to update > +the Access and Dirty flags, for example) never results in a > +KVM_EXIT_MMIO exit when the slot has the KVM_MEM_READONLY flag. This > +is because KVM cannot provide the data that would be written by the > +page-table walker, making it impossible to emulate the access. > +Instead, an abort (data abort if the cause of the page-table update > +was a load or a store, instruction abort if it was an instruction > +fetch) is injected in the guest. > > 4.36 KVM_SET_TSS_ADDR > --------------------- -- Best, Oliver