From: Oliver Upton <oliver.upton@linux.dev>
To: Sean Christopherson <seanjc@google.com>
Cc: Anish Moorthy <amoorthy@google.com>,
jthoughton@google.com, kvm@vger.kernel.org
Subject: Re: [WIP Patch v2 09/14] KVM: Introduce KVM_CAP_MEMORY_FAULT_NOWAIT without implementation
Date: Tue, 21 Mar 2023 20:23:06 +0000 [thread overview]
Message-ID: <ZBoSKm3CUoBC0l5X@linux.dev> (raw)
In-Reply-To: <ZBnEO5l7hZMlhi/1@google.com>
On Tue, Mar 21, 2023 at 07:50:35AM -0700, Sean Christopherson wrote:
> On Mon, Mar 20, 2023, Oliver Upton wrote:
> > On Fri, Mar 17, 2023 at 01:17:22PM -0700, Sean Christopherson wrote:
> > > On Fri, Mar 17, 2023, Oliver Upton wrote:
> > > > I'm not a fan of this architecture-specific dependency. Userspace is already
> > > > explicitly opting in to this behavior by way of the memslot flag. These sort
> > > > of exits are entirely orthogonal to the -EFAULT conversion earlier in the
> > > > series.
> > >
> > > Ya, yet another reason not to speculate on why KVM wasn't able to resolve a fault.
> >
> > Regardless of what we name this memslot flag, we're already getting explicit
> > opt-in from userspace for new behavior. There seems to be zero value in
> > supporting memslot_flag && !MEMORY_FAULT_EXIT (i.e. returning EFAULT),
> > so why even bother?
>
> Because there are use cases for MEMORY_FAULT_EXIT beyond fast-only gup.
To be abundantly clear -- I have no issue with (nor care about) the other
MEMORY_FAULT_EXIT changes. If we go the route of explicit user opt-in then
that deserves its own distinct bit of UAPI. None of my objection pertains
to the conversion of existing -EFAULT exits.
> We could have the memslot feature depend on the MEMORY_FAULT_EXIT capability,
> but I don't see how that adds value for either KVM or userspace.
That is exactly what I want to avoid! My issue was the language here:
+(*) NOTE: On x86, KVM_CAP_X86_MEMORY_FAULT_EXIT must be enabled for the
+KVM_MEMFAULT_REASON_ABSENT_MAPPING_reason: otherwise userspace will only receive
+a -EFAULT from KVM_RUN without any useful information.
Which sounds to me as though there are *two* UAPI bits for the whole fast-gup
failed interaction (flip a bit in the CAP and set a bit on the memslot, but
only for x86).
What I'm asking for is this:
1) A capability advertising MEMORY_FAULT_EXIT to userspace. Either usurp
EFAULT or require userspace to enable this capability to convert
_existing_ EFAULT exits to the new way of the world.
2) A capability and a single memslot flag to enable the fast-gup-only
behavior (naming TBD). This does not depend on (1) in any way, i.e.
only setting (2) should still result in MEMORY_FAULT_EXITs when fast
gup fails. IOW, enabling (2) should always yield precise fault
information to userspace.
--
Thanks,
Oliver
next prev parent reply other threads:[~2023-03-21 20:24 UTC|newest]
Thread overview: 60+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-15 2:17 [WIP Patch v2 00/14] Avoiding slow get-user-pages via memory fault exit Anish Moorthy
2023-03-15 2:17 ` [WIP Patch v2 01/14] KVM: selftests: Allow many vCPUs and reader threads per UFFD in demand paging test Anish Moorthy
2023-03-15 2:17 ` [WIP Patch v2 02/14] KVM: selftests: Use EPOLL in userfaultfd_util reader threads and signal errors via TEST_ASSERT Anish Moorthy
2023-03-15 2:17 ` [WIP Patch v2 03/14] KVM: Allow hva_pfn_fast to resolve read-only faults Anish Moorthy
2023-03-15 2:17 ` [WIP Patch v2 04/14] KVM: x86: Add KVM_CAP_X86_MEMORY_FAULT_EXIT and associated kvm_run field Anish Moorthy
2023-03-17 0:02 ` Isaku Yamahata
2023-03-17 18:33 ` Anish Moorthy
2023-03-17 19:30 ` Oliver Upton
2023-03-17 21:50 ` Sean Christopherson
2023-03-17 22:44 ` Anish Moorthy
2023-03-20 15:53 ` Sean Christopherson
2023-03-20 18:19 ` Anish Moorthy
2023-03-20 22:11 ` Anish Moorthy
2023-03-21 15:21 ` Sean Christopherson
2023-03-21 18:01 ` Anish Moorthy
2023-03-21 19:43 ` Sean Christopherson
2023-03-22 21:06 ` Anish Moorthy
2023-03-22 23:17 ` Sean Christopherson
2023-03-28 22:19 ` Anish Moorthy
2023-04-04 19:34 ` Sean Christopherson
2023-04-04 20:40 ` Anish Moorthy
2023-04-04 22:07 ` Sean Christopherson
2023-04-05 20:21 ` Anish Moorthy
2023-03-17 18:35 ` Oliver Upton
2023-03-15 2:17 ` [WIP Patch v2 05/14] KVM: x86: Implement memory fault exit for direct_map Anish Moorthy
2023-03-15 2:17 ` [WIP Patch v2 06/14] KVM: x86: Implement memory fault exit for kvm_handle_page_fault Anish Moorthy
2023-03-15 2:17 ` [WIP Patch v2 07/14] KVM: x86: Implement memory fault exit for setup_vmgexit_scratch Anish Moorthy
2023-03-15 2:17 ` [WIP Patch v2 08/14] KVM: x86: Implement memory fault exit for FNAME(fetch) Anish Moorthy
2023-03-15 2:17 ` [WIP Patch v2 09/14] KVM: Introduce KVM_CAP_MEMORY_FAULT_NOWAIT without implementation Anish Moorthy
2023-03-17 18:59 ` Oliver Upton
2023-03-17 20:15 ` Anish Moorthy
2023-03-17 20:54 ` Sean Christopherson
2023-03-17 23:42 ` Anish Moorthy
2023-03-20 15:13 ` Sean Christopherson
2023-03-20 19:53 ` Anish Moorthy
2023-03-17 20:17 ` Sean Christopherson
2023-03-20 22:22 ` Oliver Upton
2023-03-21 14:50 ` Sean Christopherson
2023-03-21 20:23 ` Oliver Upton [this message]
2023-03-21 21:01 ` Sean Christopherson
2023-03-15 2:17 ` [WIP Patch v2 10/14] KVM: x86: Implement KVM_CAP_MEMORY_FAULT_NOWAIT Anish Moorthy
2023-03-17 0:32 ` Isaku Yamahata
2023-03-15 2:17 ` [WIP Patch v2 11/14] KVM: arm64: Allow user_mem_abort to return 0 to signal a 'normal' exit Anish Moorthy
2023-03-17 18:18 ` Oliver Upton
2023-03-15 2:17 ` [WIP Patch v2 12/14] KVM: arm64: Implement KVM_CAP_MEMORY_FAULT_NOWAIT Anish Moorthy
2023-03-17 18:27 ` Oliver Upton
2023-03-17 19:00 ` Anish Moorthy
2023-03-17 19:03 ` Oliver Upton
2023-03-17 19:24 ` Sean Christopherson
2023-03-15 2:17 ` [WIP Patch v2 13/14] KVM: selftests: Add memslot_flags parameter to memstress_create_vm Anish Moorthy
2023-03-15 2:17 ` [WIP Patch v2 14/14] KVM: selftests: Handle memory fault exits in demand_paging_test Anish Moorthy
2023-03-17 17:43 ` [WIP Patch v2 00/14] Avoiding slow get-user-pages via memory fault exit Oliver Upton
2023-03-17 18:13 ` Sean Christopherson
2023-03-17 18:46 ` David Matlack
2023-03-17 18:54 ` Oliver Upton
2023-03-17 18:59 ` David Matlack
2023-03-17 19:53 ` Anish Moorthy
2023-03-17 22:03 ` Sean Christopherson
2023-03-20 15:56 ` Sean Christopherson
2023-03-17 20:35 ` Sean Christopherson
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