From: Oliver Upton <oliver.upton@linux.dev>
To: Sean Christopherson <seanjc@google.com>
Cc: Anish Moorthy <amoorthy@google.com>,
maz@kernel.org, kvm@vger.kernel.org, kvmarm@lists.linux.dev,
robert.hoo.linux@gmail.com, jthoughton@google.com,
dmatlack@google.com, axelrasmussen@google.com, peterx@redhat.com,
nadav.amit@gmail.com, isaku.yamahata@gmail.com,
kconsul@linux.vnet.ibm.com
Subject: Re: [PATCH v7 08/14] KVM: arm64: Enable KVM_CAP_MEMORY_FAULT_INFO and annotate fault in the stage-2 fault handler
Date: Mon, 4 Mar 2024 21:03:59 +0000 [thread overview]
Message-ID: <ZeY3P8Za5Q6pkkQV@linux.dev> (raw)
In-Reply-To: <ZeYv86atkVpVMa2S@google.com>
On Mon, Mar 04, 2024 at 12:32:51PM -0800, Sean Christopherson wrote:
> On Mon, Mar 04, 2024, Oliver Upton wrote:
> > On Mon, Mar 04, 2024 at 08:00:15PM +0000, Oliver Upton wrote:
[...]
> > Duh, kvm_vcpu_trap_is_exec_fault() (not to be confused with
> > kvm_vcpu_trap_is_iabt()) filters for S1PTW, so this *should*
> > shake out as a write fault on the stage-1 descriptor.
> >
> > With that said, an architecture-neutral UAPI may not be able to capture
> > the nuance of a fault. This UAPI will become much more load-bearing in
> > the future, and the loss of granularity could become an issue.
>
> What is the possible fallout from loss of granularity/nuance? E.g. if the worst
> case scenario is that KVM may exit to userspace multiple times in order to resolve
> the problem, IMO that's an acceptable cost for having "dumb", common uAPI.
>
> The intent/contract of the exit to userspace isn't for userspace to be able to
> completely understand what fault occurred, but rather for KVM to communicate what
> action userspace needs to take in order for KVM to make forward progress.
For one, the stage-2 page tables can describe permissions beyond RWX.
MTE tag allocation can be controlled at stage-2, which (confusingly)
desribes if the guest can insert tags in an opaque, physical space not
described by HPFAR.
There is a corresponding bit in ESR_EL2 that describes this at the time
of a fault, and R/W/X flags aren't enough to convey the right corrective
action.
> > Marc had some ideas about forwarding the register state to userspace
> > directly, which should be the right level of information for _any_ fault
> > taken to userspace.
>
> I don't know enough about ARM to weigh in on that side of things, but for x86
> this definitely doesn't hold true.
We tend to directly model the CPU architecture wherever possible, as it
is the only way to create something intelligible. That same rationale
applies to a huge portion of KVM UAPI; it is architecture-dependent by
design.
> E.g. on the x86 side, KVM intentionally sets
> reserved bits in SPTEs for "caching" emulated MMIO accesses, and the resulting
> fault captures the "reserved bits set" information in register state. But that's
> purely an (optional) imlementation detail of KVM that should never be exposed to
> userspace.
MMIO accesses would show up elsewhere though, right? If these magic
SPTEs were causing -EFAULT exits then something must've gone sideways.
Either way, I have no issues whatsoever if the direction for x86 is to
provide abstracted fault information.
--
Thanks,
Oliver
next prev parent reply other threads:[~2024-03-04 21:04 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-15 23:53 [PATCH v7 00/14] Improve KVM + userfaultfd performance via KVM_EXIT_MEMORY_FAULTs on stage-2 faults Anish Moorthy
2024-02-15 23:53 ` [PATCH v7 01/14] KVM: Clarify meaning of hva_to_pfn()'s 'atomic' parameter Anish Moorthy
2024-02-15 23:53 ` [PATCH v7 02/14] KVM: Add function comments for __kvm_read/write_guest_page() Anish Moorthy
2024-02-15 23:53 ` [PATCH v7 03/14] KVM: Documentation: Make note of the KVM_MEM_GUEST_MEMFD memslot flag Anish Moorthy
2024-04-09 22:47 ` Sean Christopherson
2024-02-15 23:53 ` [PATCH v7 04/14] KVM: Simplify error handling in __gfn_to_pfn_memslot() Anish Moorthy
2024-04-09 22:44 ` Sean Christopherson
2024-02-15 23:53 ` [PATCH v7 05/14] KVM: Define and communicate KVM_EXIT_MEMORY_FAULT RWX flags to userspace Anish Moorthy
2024-02-15 23:53 ` [PATCH v7 06/14] KVM: Add memslot flag to let userspace force an exit on missing hva mappings Anish Moorthy
2024-03-08 22:07 ` Sean Christopherson
2024-03-09 0:46 ` David Matlack
2024-03-11 4:45 ` Oliver Upton
2024-03-11 16:20 ` David Matlack
2024-03-11 16:36 ` Sean Christopherson
2024-03-11 17:08 ` Anish Moorthy
2024-03-11 21:21 ` Oliver Upton
2024-02-15 23:53 ` [PATCH v7 07/14] KVM: x86: Enable KVM_CAP_EXIT_ON_MISSING and annotate EFAULTs from stage-2 fault handler Anish Moorthy
2024-02-15 23:53 ` [PATCH v7 08/14] KVM: arm64: Enable KVM_CAP_MEMORY_FAULT_INFO and annotate fault in the " Anish Moorthy
2024-03-04 20:00 ` Oliver Upton
2024-03-04 20:10 ` Oliver Upton
2024-03-04 20:32 ` Sean Christopherson
2024-03-04 21:03 ` Oliver Upton [this message]
2024-03-04 22:49 ` Sean Christopherson
2024-03-05 1:01 ` Oliver Upton
2024-03-05 15:39 ` Sean Christopherson
2024-02-15 23:54 ` [PATCH v7 09/14] KVM: arm64: Implement and advertise KVM_CAP_EXIT_ON_MISSING Anish Moorthy
2024-02-15 23:54 ` [PATCH v7 10/14] KVM: selftests: Report per-vcpu demand paging rate from demand paging test Anish Moorthy
2024-04-09 22:49 ` Sean Christopherson
2024-02-15 23:54 ` [PATCH v7 11/14] KVM: selftests: Allow many vCPUs and reader threads per UFFD in " Anish Moorthy
2024-04-09 22:58 ` Sean Christopherson
2024-02-15 23:54 ` [PATCH v7 12/14] KVM: selftests: Use EPOLL in userfaultfd_util reader threads and signal errors via TEST_ASSERT Anish Moorthy
2024-02-15 23:54 ` [PATCH v7 13/14] KVM: selftests: Add memslot_flags parameter to memstress_create_vm() Anish Moorthy
2024-02-15 23:54 ` [PATCH v7 14/14] KVM: selftests: Handle memory fault exits in demand_paging_test Anish Moorthy
2024-02-16 7:36 ` [PATCH v7 00/14] Improve KVM + userfaultfd performance via KVM_EXIT_MEMORY_FAULTs on stage-2 faults Gupta, Pankaj
2024-02-16 20:00 ` Anish Moorthy
2024-02-16 23:40 ` Axel Rasmussen
2024-02-21 7:35 ` Gupta, Pankaj
2024-04-10 0:19 ` Sean Christopherson
2024-05-07 17:38 ` Anish Moorthy
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