These patches are based on kvm/next, and are also available at:
https://github.com/mdroth/linux/commits/sev-selftests-ucall-rfc1
== BACKGROUND ==
These patches are a prerequisite for adding selftest support for SEV guests and possibly other confidential computing implementations in the future. They were motivated by a suggestion Paolo made in response to the initial SEV selftest RFC:
https://lore.kernel.org/lkml/20211025035833.yqphcnf5u3lk4zgg@amd.com/T/#m959...
Since the changes touch multiple archs and ended up creating a bit more churn than expected, I thought it would be a good idea to carve this out into a separate standalone series for reviewers who may be more interested in the ucall changes than anything SEV-related.
To summarize, x86 relies on a ucall based on using PIO intructions to generate an exit to userspace and provide the GVA of a dynamically-allocated ucall struct that resides in guest memory and contains information about how to handle/interpret the exit. This doesn't work for SEV guests for 3 main reasons:
1) The guest memory is generally encrypted during run-time, so the guest needs to ensure the ucall struct is allocated in shared memory. 2) The guest page table is also encrypted, so the address would need to be a GPA instead of a GVA. 3) The guest vCPU register may also be encrypted in the case of SEV-ES/SEV-SNP, so the approach of examining vCPU register state has additional requirements such as requiring guest code to implement a #VC handler that can provide the appropriate registers via a vmgexit.
To address these issues, the SEV selftest RFC1 patchset introduced a set of new SEV-specific interfaces that closely mirrored the functionality of ucall()/get_ucall(), but relied on a pre-allocated/static ucall buffer in shared guest memory so it that guest code could pass messages/state to the host by simply writing to this pre-arranged shared memory region and then generating an exit to userspace (via a halt instruction).
Paolo suggested instead implementing support for test/guest-specific ucall implementations that could be used as an alternative to the default PIO-based ucall implementations as-needed based on test/guest requirements, while still allowing for tests to use a common set interfaces like ucall()/get_ucall().
== OVERVIEW ==
This series implements the above functionality by introducing a new ucall_ops struct that can be used to register a particular ucall implementation as need, then re-implements x86/arm64/s390x in terms of the ucall_ops.
But for the purposes of introducing a new ucall_ops implementation appropriate for SEV, there are a couple issues that resulted in the need for some additional ucall interfaces as well:
a) ucall() doesn't take a pointer to the ucall struct it modifies, so to make it work in the case of an implementation that relies a pre-allocated ucall struct in shared guest memory some sort of global lookup functionality would be needed to locate the appropriate ucall struct for a particular VM/vcpu combination, and this would need to be made accessible for use by the guest as well. guests would then need some way of determining what VM/vcpu identifiers they need to use to do the lookup, which to do reliably would likely require seeding the guest with those identifiers in advance, which is possible, but much more easily achievable by simply adding a ucall() alternative that accepts a pointer to the ucall struct for that particular VM/vcpu.
b) get_ucall() *does* take a pointer to a ucall struct, but currently zeroes it out and uses it to copy the guest's ucall struct into. It *could* be re-purposed to handle the case where the pointer is an actual pointer to the ucall struct in shared guest memory, but that could cause problems since callers would need some idea of what the underlying ucall implementation expects. Ideally the interfaces would be agnostic to the ucall implementation.
So to address those issues, this series also allows ucall implementations to optionally be extended to support a set of 'shared' ops that are used in the following manner:
host: uc_gva = ucall_shared_alloc() setup_vm_args(vm, uc_gva)
guest: ucall_shared(uc_gva, ...)
host: uget_ucall_shared(uc_gva, ...)
and then implements a new ucall implementation, ucall_ops_halt, based around these shared interfaces and halt instructions.
While this doesn't really meet the initial goal of re-using the existing ucall interfaces as-is, the hope is that these *_shared interfaces are general enough to be re-usable things other than SEV, or at least improve on code readability over the initial SEV-specific interfaces.
Any review/comments are greatly appreciated!
---------------------------------------------------------------- Michael Roth (10): kvm: selftests: move base kvm_util.h declarations to kvm_util_base.h kvm: selftests: move ucall declarations into ucall_common.h kvm: selftests: introduce ucall_ops for test/arch-specific ucall implementations kvm: arm64: selftests: use ucall_ops to define default ucall implementation (COMPILE-TESTED ONLY) kvm: s390: selftests: use ucall_ops to define default ucall implementation kvm: selftests: add ucall interfaces based around shared memory kvm: selftests: add ucall_shared ops for PIO kvm: selftests: introduce ucall implementation based on halt instructions kvm: selftests: add GUEST_SHARED_* macros for shared ucall implementations kvm: selftests: add ucall_test to test various ucall functionality
tools/testing/selftests/kvm/.gitignore | 1 + tools/testing/selftests/kvm/Makefile | 5 +- .../testing/selftests/kvm/include/aarch64/ucall.h | 18 + tools/testing/selftests/kvm/include/kvm_util.h | 408 +-------------------- .../testing/selftests/kvm/include/kvm_util_base.h | 368 +++++++++++++++++++ tools/testing/selftests/kvm/include/s390x/ucall.h | 18 + tools/testing/selftests/kvm/include/ucall_common.h | 147 ++++++++ tools/testing/selftests/kvm/include/x86_64/ucall.h | 19 + tools/testing/selftests/kvm/lib/aarch64/ucall.c | 43 +-- tools/testing/selftests/kvm/lib/s390x/ucall.c | 45 +-- tools/testing/selftests/kvm/lib/ucall_common.c | 133 +++++++ tools/testing/selftests/kvm/lib/x86_64/ucall.c | 82 +++-- tools/testing/selftests/kvm/ucall_test.c | 182 +++++++++ 13 files changed, 982 insertions(+), 487 deletions(-) create mode 100644 tools/testing/selftests/kvm/include/aarch64/ucall.h create mode 100644 tools/testing/selftests/kvm/include/kvm_util_base.h create mode 100644 tools/testing/selftests/kvm/include/s390x/ucall.h create mode 100644 tools/testing/selftests/kvm/include/ucall_common.h create mode 100644 tools/testing/selftests/kvm/include/x86_64/ucall.h create mode 100644 tools/testing/selftests/kvm/lib/ucall_common.c create mode 100644 tools/testing/selftests/kvm/ucall_test.c