Re: [RFC PATCH v3 00/29] KVM: arm64: Make CPU ID registers writable by userspace

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi Reiji,

On Tue, Nov 23, 2021 at 09:13:27PM -0800, Reiji Watanabe wrote:
> HI Alex,
> 
> On Tue, Nov 23, 2021 at 7:59 AM Alexandru Elisei
> <alexandru.elisei@arm.com> wrote:
> >
> > Hi Reiji,
> >
> > I started reviewing the series, but I ended up being very confused, see
> > below.
> >
> > 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.
> >
> > What not use VCPU features? Isn't that why the field
> > kvm_vcpu_init->features exists in the first place? This cover letter does
> > nothing to explaing why any changes are needed.
> >
> > Do you require finer grained control over certain feature that you cannot
> > get with the 32 * 7 = 224 feature flag bits from kvm_vcpu_init? Does using
> > the ID registers simplify certain aspects of the implementation?
> 
> Since some features are not binary in nature (e.g. AA64DFR0_EL1.BRPs
> fields indicate number of breakpoints minus 1), using
> kvm_vcpu_init->features to configure such features is inconvenient.

I see, this makes a lot of sense and this looks like a nice solution to
that problem.

> 
> One of the reasons why we want the finer grained control is that
> we want to expose a uniform set/level of features for a group of
> guests on systems with different ARM CPUs.

So here you are talking specifically about KVM not checking that all VCPUs
have the same feature bits set in vcpu->arch.features, which makes it
possible for userspace to set different features for different VCPUs,
right?

Thanks,
Alex

> 
> I will update the cover letter.
> 
> Thanks,
> Reiji
> 
> >
> > Thanks,
> > Alex
> >
> > >
> > > 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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