[RFC PATCH v3 00/29] KVM: arm64: Make CPU ID registers writable by userspace
Alexandru Elisei
alexandru.elisei at arm.com
Wed Nov 24 02:48:35 PST 2021
Hi Reiji,
On Tue, Nov 23, 2021 at 09:49:02PM -0800, Reiji Watanabe wrote:
> Hi Alex,
>
> On Tue, Nov 23, 2021 at 8:25 AM Alexandru Elisei
> <alexandru.elisei at arm.com> wrote:
> >
> > Hi Reiji,
> >
> > The API documentation for KVM_ARM_VCPU_INIT states:
> >
> > "Userspace can call this function multiple times for a given vcpu,
> > including after the vcpu has been run. This will reset the vcpu to its
> > initial state. All calls to this function after the initial call must use
> > the same target and same set of feature flags, otherwise EINVAL will be
> > returned."
> >
> > The consequences of that, according to my understanding:
> >
> > 1. Any changes to the VCPU features made by KVM are observable by
> > userspace.
> >
> > 2. The features in KVM weren't designed and implemented to be disabled
> > after being enabled.
> >
> > With that in mind, I have two questions:
> >
> > 1. What happens when userspace disables a feature via the ID registers
> > which is set in vcpu->arch.features? Does the feature bit get cleared from
> > vcpu->arch.features? Does it stay set? If it gets cleared, is it now
> > possible for userspace to call KVM_ARM_VCPU_INIT again with a different set
> > of VCPU features (it doesn't look possible to me after looking at the
> > code). If it stays set, what does it mean when userspace calls
> > KVM_ARM_VCPU_INIT with a different set of features enabled than what is
> > present in the ID registers? Should the ID registers be changed to match
> > the features that userspace set in the last KVM_ARM_VCPU_INIT call (it
> > looks to me that the ID registers are not changed)?
>
> KVM will not allow userspace to set the ID register value that conflicts
> with CPU features that are configured by the initial KVM_ARM_VCPU_INIT
> (or there are a few more APIs).
> KVM_SET_ONE_REG for such requests will fail.
>
>
> > 2. What happens to vcpu->arch.features when userspace enables a feature via
> > the ID registers which is not present in the bitmap?
>
> The answer for this question is basically the same as above.
> Userspace is not allowed to enable a feature via the ID registers
> which is not present in the bit map.
>
> The cover lever included a brief explanation of this, but I will
> try to improve the explanation:-)
So the ID registers are used to set the version of a feature which is present in
the VCPU features bitmap, not to enable or a disable a VCPU feature. Thank you
for the explanation!
Thanks,
Alex
>
> Regards,
> Reiji
>
> >
> > Thanks,
> > Alex
> >
> > On Tue, Nov 16, 2021 at 10:43:30PM -0800, Reiji Watanabe wrote:
> > > In KVM/arm64, values of ID registers for a guest are mostly same as
> > > its host's values except for bits for feature that KVM doesn't support
> > > and for opt-in features that userspace didn't configure. Userspace
> > > can use KVM_SET_ONE_REG to a set ID register value, but it fails
> > > if userspace attempts to modify the register value.
> > >
> > > This patch series adds support to allow userspace to modify a value of
> > > ID registers (as long as KVM can support features that are indicated
> > > in the registers) so userspace can have more control of configuring
> > > and unconfiguring features for guests.
> > >
> > > The patch series is for both VHE and non-VHE, except for protected VMs,
> > > which have a different way of configuring ID registers based on its
> > > different requirements [1].
> > > There was a patch series that tried to achieve the same thing [2].
> > > A few snippets of codes in this series were inspired by or came from [2].
> > >
> > > The initial value of ID registers for a vCPU will be the host's value
> > > with bits cleared for unsupported features and for opt-in features that
> > > were not configured. So, the initial value userspace can see (via
> > > KVM_GET_ONE_REG) is the upper limit that can be set for the register.
> > > Any requests to change the value that conflicts with opt-in features'
> > > configuration will fail.
> > >
> > > When a guest tries to use a CPU feature that is not exposed to the guest,
> > > trapping it (to emulate a real CPU's behavior) would generally be a
> > > desirable behavior (when it is possible with no or little side effects).
> > > The later patches in the series add codes for this. Only features that
> > > can be trapped independently will be trapped by this series though.
> > >
> > > This series adds kunit tests for new functions in sys_regs.c (except for
> > > trivial ones), and these tests are enabled with a new configuration
> > > option 'CONFIG_KVM_KUNIT_TEST'.
> > >
> > > The series is based on v5.16-rc1.
> > >
> > > v3:
> > > - Remove ID register consistency checking across vCPUs [Oliver]
> > > - Change KVM_CAP_ARM_ID_REG_WRITABLE to
> > > KVM_CAP_ARM_ID_REG_CONFIGURABLE [Oliver]
> > > - Add KUnit testing for ID register validation and trap initialization.
> > > - Change read_id_reg() to take care of ID_AA64PFR0_EL1.GIC
> > > - Add a helper of read_id_reg() (__read_id_reg()) and use the helper
> > > instead of directly using __vcpu_sys_reg()
> > > - Change not to run kvm_id_regs_consistency_check() and
> > > kvm_vcpu_init_traps() for protected VMs.
> > > - Update selftest to remove test cases for ID register consistency
> > > checking across vCPUs and to add test cases for ID_AA64PFR0_EL1.GIC.
> > >
> > > v2: https://lore.kernel.org/all/20211103062520.1445832-1-reijiw@google.com/
> > > - Remove unnecessary line breaks. [Andrew]
> > > - Use @params for comments. [Andrew]
> > > - Move arm64_check_features to arch/arm64/kvm/sys_regs.c and
> > > change that KVM specific feature check function. [Andrew]
> > > - Remove unnecessary raz handling from __set_id_reg. [Andrew]
> > > - Remove sys_val field from the initial id_reg_info and add it
> > > in the later patch. [Andrew]
> > > - Call id_reg->init() from id_reg_info_init(). [Andrew]
> > > - Fix cpuid_feature_cap_perfmon_field() to convert 0xf to 0x0
> > > (and use it in the following patches).
> > > - Change kvm_vcpu_first_run_init to set has_run_once to false
> > > when kvm_id_regs_consistency_check() fails.
> > > - Add a patch to introduce id_reg_info for ID_AA64MMFR0_EL1,
> > > which requires special validity checking for TGran*_2 fields.
> > > - Add patches to introduce id_reg_info for ID_DFR1_EL1 and
> > > ID_MMFR0_EL1, which are required due to arm64_check_features
> > > implementation change.
> > > - Add a new argument, which is a pointer to id_reg_info, for
> > > id_reg_info's validate()
> > >
> > > v1: https://lore.kernel.org/all/20211012043535.500493-1-reijiw@google.com/
> > >
> > > [1] https://lore.kernel.org/kvmarm/20211010145636.1950948-1-tabba@google.com/
> > > [2] https://lore.kernel.org/kvm/20201102033422.657391-1-liangpeng10@huawei.com/
> > >
> > > Reiji Watanabe (29):
> > > KVM: arm64: Add has_reset_once flag for vcpu
> > > KVM: arm64: Save ID registers' sanitized value per vCPU
> > > KVM: arm64: Introduce struct id_reg_info
> > > KVM: arm64: Make ID_AA64PFR0_EL1 writable
> > > KVM: arm64: Make ID_AA64PFR1_EL1 writable
> > > KVM: arm64: Make ID_AA64ISAR0_EL1 writable
> > > KVM: arm64: Make ID_AA64ISAR1_EL1 writable
> > > KVM: arm64: Make ID_AA64MMFR0_EL1 writable
> > > KVM: arm64: Hide IMPLEMENTATION DEFINED PMU support for the guest
> > > KVM: arm64: Make ID_AA64DFR0_EL1 writable
> > > KVM: arm64: Make ID_DFR0_EL1 writable
> > > KVM: arm64: Make ID_DFR1_EL1 writable
> > > KVM: arm64: Make ID_MMFR0_EL1 writable
> > > KVM: arm64: Make MVFR1_EL1 writable
> > > KVM: arm64: Make ID registers without id_reg_info writable
> > > KVM: arm64: Add consistency checking for frac fields of ID registers
> > > KVM: arm64: Introduce KVM_CAP_ARM_ID_REG_CONFIGURABLE capability
> > > KVM: arm64: Add kunit test for ID register validation
> > > KVM: arm64: Use vcpu->arch cptr_el2 to track value of cptr_el2 for VHE
> > > KVM: arm64: Use vcpu->arch.mdcr_el2 to track value of mdcr_el2
> > > KVM: arm64: Introduce framework to trap disabled features
> > > KVM: arm64: Trap disabled features of ID_AA64PFR0_EL1
> > > KVM: arm64: Trap disabled features of ID_AA64PFR1_EL1
> > > KVM: arm64: Trap disabled features of ID_AA64DFR0_EL1
> > > KVM: arm64: Trap disabled features of ID_AA64MMFR1_EL1
> > > KVM: arm64: Trap disabled features of ID_AA64ISAR1_EL1
> > > KVM: arm64: Initialize trapping of disabled CPU features for the guest
> > > KVM: arm64: Add kunit test for trap initialization
> > > KVM: arm64: selftests: Introduce id_reg_test
> > >
> > > Documentation/virt/kvm/api.rst | 8 +
> > > arch/arm64/include/asm/cpufeature.h | 2 +-
> > > arch/arm64/include/asm/kvm_arm.h | 32 +
> > > arch/arm64/include/asm/kvm_host.h | 15 +
> > > arch/arm64/include/asm/sysreg.h | 2 +
> > > arch/arm64/kvm/Kconfig | 11 +
> > > arch/arm64/kvm/arm.c | 12 +-
> > > arch/arm64/kvm/debug.c | 13 +-
> > > arch/arm64/kvm/hyp/vhe/switch.c | 14 +-
> > > arch/arm64/kvm/reset.c | 4 +
> > > arch/arm64/kvm/sys_regs.c | 1265 +++++++++++++++--
> > > arch/arm64/kvm/sys_regs_test.c | 1109 +++++++++++++++
> > > include/uapi/linux/kvm.h | 1 +
> > > tools/arch/arm64/include/asm/sysreg.h | 1 +
> > > tools/testing/selftests/kvm/.gitignore | 1 +
> > > tools/testing/selftests/kvm/Makefile | 1 +
> > > .../selftests/kvm/aarch64/id_reg_test.c | 1128 +++++++++++++++
> > > 17 files changed, 3488 insertions(+), 131 deletions(-)
> > > create mode 100644 arch/arm64/kvm/sys_regs_test.c
> > > create mode 100644 tools/testing/selftests/kvm/aarch64/id_reg_test.c
> > >
> > > --
> > > 2.34.0.rc1.387.gb447b232ab-goog
> > >
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