Hi Sean, here's what I'm planing to send up as v2 of the scalable
userfaultfd series.
Don't worry, I'm not asking you to review this all :) I just have a few
remaining questions regarding KVM_CAP_MEMORY_FAULT_EXIT which seem important
enough to mention before I ask for more attention from others, and they'll be
clearer with the patches in hand. Anything else I'm happy to find out about when
I send the actual v2.
I want your opinion on
1. The general API I've set up for KVM_CAP_MEMORY_FAULT_EXIT
(described in the api.rst file)
2. Whether the UNKNOWN exit reason cases (everywhere but
handle_error_pfn atm) would need to be given "real" reasons
before this could be merged.
3. If you think I've missed sites that currently -EFAULT to userspace
About (3): after we agreed to only tackle cases where -EFAULT currently makes it
to userspace, I went though our list and tried to trace which EFAULTS actually
bubble up to KVM_RUN. That set ended being suspiciously small, so I wanted to
sanity-check my findings with you. Lmk if you see obvious errors in my list
below.
--- EFAULTs under KVM_RUN ---
Confident that needs conversion (already converted)
---------------------------------------------------
* direct_map
* handle_error_pfn
* setup_vmgexit_scratch
* kvm_handle_page_fault
* FNAME(fetch)
EFAULT does not propagate to userspace (do not convert)
-------------------------------------------------------
* record_steal_time (arch/x86/kvm/x86.c:3463)
* hva_to_pfn_retry
* kvm_vcpu_map
* FNAME(update_accessed_dirty_bits)
* __kvm_gfn_to_hva_cache_init
Might actually make it to userspace, but only through
kvm_read|write_guest_offset_cached- would be covered by those conversions
* kvm_gfn_to_hva_cache_init
* __kvm_read_guest_page
* hva_to_pfn_remapped
handle_error_pfn will handle this for the scalable uffd case. Don't think
other callers -EFAULT to userspace.
Still unsure if needs conversion
--------------------------------
* __kvm_read_guest_atomic
The EFAULT might be propagated though FNAME(sync_page)?
* kvm_write_guest_offset_cached (virt/kvm/kvm_main.c:3226)
* __kvm_write_guest_page
Called from kvm_write_guest_offset_cached: if that needs change, this does too
* kvm_write_guest_page
Two interesting paths:
- kvm_pv_clock_pairing returns a custom KVM_EFAULT error here
(arch/x86/kvm/x86.c:9578)
- kvm_write_guest_offset_cached returns this directly (so if that needs
change, this does too)
* kvm_read_guest_offset_cached
I actually do see a path to userspace, but it's through hyper-v, which we've
said is out of scope for round 1.
--- Actual Cover Letter ---
Omitted: hasn't changed much since v1 anyways
--- Changelog ---
WIP v2
- Introduce KVM_CAP_X86_MEMORY_FAULT_EXIT.
- API changes:
- Gate KVM_CAP_MEMORY_FAULT_NOWAIT behind
KVM_CAP_x86_MEMORY_FAULT_EXIT (on x86 only: arm has no such
requirement).
- Switched to memslot flag
- Take Oliver's simplification to the "allow fast gup for readable
faults" logic.
- Slightly redefine the return code of user_mem_abort.
- Fix documentation errors brought up by Marc
- Reword commit messages in imperative mood
v1: https://lore.kernel.org/kvm/20230215011614.725983-1-amoorthy@xxxxxxxxxx/
Anish Moorthy (14):
KVM: selftests: Allow many vCPUs and reader threads per UFFD in demand
paging test
KVM: selftests: Use EPOLL in userfaultfd_util reader threads and
signal errors via TEST_ASSERT
KVM: Allow hva_pfn_fast to resolve read-only faults.
KVM: x86: Add KVM_CAP_X86_MEMORY_FAULT_EXIT and associated kvm_run
field
KVM: x86: Implement memory fault exit for direct_map
KVM: x86: Implement memory fault exit for kvm_handle_page_fault
KVM: x86: Implement memory fault exit for setup_vmgexit_scratch
KVM: x86: Implement memory fault exit for FNAME(fetch)
KVM: Introduce KVM_CAP_MEMORY_FAULT_NOWAIT without implementation
KVM: x86: Implement KVM_CAP_MEMORY_FAULT_NOWAIT
KVM: arm64: Allow user_mem_abort to return 0 to signal a 'normal' exit
KVM: arm64: Implement KVM_CAP_MEMORY_FAULT_NOWAIT
KVM: selftests: Add memslot_flags parameter to memstress_create_vm
KVM: selftests: Handle memory fault exits in demand_paging_test
Documentation/virt/kvm/api.rst | 74 ++++-
arch/arm64/kvm/arm.c | 1 +
arch/arm64/kvm/mmu.c | 29 +-
arch/x86/kvm/mmu/mmu.c | 42 ++-
arch/x86/kvm/mmu/paging_tmpl.h | 4 +-
arch/x86/kvm/svm/sev.c | 4 +-
arch/x86/kvm/x86.c | 2 +
include/linux/kvm_host.h | 22 ++
include/uapi/linux/kvm.h | 19 ++
tools/include/uapi/linux/kvm.h | 17 ++
.../selftests/kvm/aarch64/page_fault_test.c | 4 +-
.../selftests/kvm/access_tracking_perf_test.c | 2 +-
.../selftests/kvm/demand_paging_test.c | 253 ++++++++++++++----
.../selftests/kvm/dirty_log_perf_test.c | 2 +-
.../testing/selftests/kvm/include/memstress.h | 2 +-
.../selftests/kvm/include/userfaultfd_util.h | 18 +-
tools/testing/selftests/kvm/lib/memstress.c | 4 +-
.../selftests/kvm/lib/userfaultfd_util.c | 160 ++++++-----
.../kvm/memslot_modification_stress_test.c | 2 +-
virt/kvm/kvm_main.c | 41 ++-
20 files changed, 544 insertions(+), 158 deletions(-)
--
2.40.0.rc1.284.g88254d51c5-goog
