Re: [RFC PATCH v4 02/26] KVM: arm64: Save ID registers' sanitized value per guest

[Reiji Watanabe <reijiw@google.com>]

Hi Fuad,

Sorry for the late reply.
I've noticed that I didn't reply to the comments in this mail.

On Mon, Jan 24, 2022 at 8:22 AM Fuad Tabba <tabba@google.com> wrote:
>
> Hi Reiji,
>
> On Thu, Jan 6, 2022 at 4:28 AM Reiji Watanabe <reijiw@google.com> wrote:
> >
> > Introduce id_regs[] in kvm_arch as a storage of guest's ID registers,
> > and save ID registers' sanitized value in the array at KVM_CREATE_VM.
> > Use the saved ones when ID registers are read by the guest or
> > userspace (via KVM_GET_ONE_REG).
> >
> > Signed-off-by: Reiji Watanabe <reijiw@google.com>
> > ---
> >  arch/arm64/include/asm/kvm_host.h | 16 ++++++++
> >  arch/arm64/kvm/arm.c              |  1 +
> >  arch/arm64/kvm/sys_regs.c         | 62 ++++++++++++++++++++++---------
> >  3 files changed, 62 insertions(+), 17 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 2a5f7f38006f..c789a0137f58 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -102,6 +102,17 @@ struct kvm_s2_mmu {
> >  struct kvm_arch_memory_slot {
> >  };
> >
> > +/*
> > + * (Op0, Op1, CRn, CRm, Op2) of ID registers is (3, 0, 0, crm, op2),
> > + * where 0<=crm<8, 0<=op2<8.
> > + */
> > +#define KVM_ARM_ID_REG_MAX_NUM 64
> > +#define IDREG_IDX(id)          ((sys_reg_CRm(id) << 3) | sys_reg_Op2(id))
> > +#define is_id_reg(id)  \
> > +       (sys_reg_Op0(id) == 3 && sys_reg_Op1(id) == 0 &&        \
> > +        sys_reg_CRn(id) == 0 && sys_reg_CRm(id) >= 0 &&        \
> > +        sys_reg_CRm(id) < 8)
> > +
>
> This is consistent with the Arm ARM "Table D12-2 System instruction
> encodings for non-Debug System register accesses".
>
> Minor nit, would it be better to have IDREG_IDX and is_id_reg in
> arch/arm64/kvm/sys_regs.h, since other similar and related ones are
> there?

I will move is_id_reg in sys_regs.c as it is used only in the file.
I will keep IDREG_IDX in arch/arm64/kvm/sys_regs.h as IDREG_IDX is
used to get an index of kvm_arch.id_regs[], which is defined in
kvm_host.h.

>
> >  struct kvm_arch {
> >         struct kvm_s2_mmu mmu;
> >
> > @@ -137,6 +148,9 @@ struct kvm_arch {
> >
> >         /* Memory Tagging Extension enabled for the guest */
> >         bool mte_enabled;
> > +
> > +       /* ID registers for the guest. */
> > +       u64 id_regs[KVM_ARM_ID_REG_MAX_NUM];
> >  };
> >
> >  struct kvm_vcpu_fault_info {
> > @@ -734,6 +748,8 @@ int kvm_arm_vcpu_arch_has_attr(struct kvm_vcpu *vcpu,
> >  long kvm_vm_ioctl_mte_copy_tags(struct kvm *kvm,
> >                                 struct kvm_arm_copy_mte_tags *copy_tags);
> >
> > +void set_default_id_regs(struct kvm *kvm);
> > +
> >  /* Guest/host FPSIMD coordination helpers */
> >  int kvm_arch_vcpu_run_map_fp(struct kvm_vcpu *vcpu);
> >  void kvm_arch_vcpu_load_fp(struct kvm_vcpu *vcpu);
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index e4727dc771bf..5f497a0af254 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -156,6 +156,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
> >         kvm->arch.max_vcpus = kvm_arm_default_max_vcpus();
> >
> >         set_default_spectre(kvm);
> > +       set_default_id_regs(kvm);
> >
> >         return ret;
> >  out_free_stage2_pgd:
> > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > index e3ec1a44f94d..80dc62f98ef0 100644
> > --- a/arch/arm64/kvm/sys_regs.c
> > +++ b/arch/arm64/kvm/sys_regs.c
> > @@ -33,6 +33,8 @@
> >
> >  #include "trace.h"
> >
> > +static u64 __read_id_reg(const struct kvm_vcpu *vcpu, u32 id);
> > +
> >  /*
> >   * All of this file is extremely similar to the ARM coproc.c, but the
> >   * types are different. My gut feeling is that it should be pretty
> > @@ -273,7 +275,7 @@ static bool trap_loregion(struct kvm_vcpu *vcpu,
> >                           struct sys_reg_params *p,
> >                           const struct sys_reg_desc *r)
> >  {
> > -       u64 val = read_sanitised_ftr_reg(SYS_ID_AA64MMFR1_EL1);
> > +       u64 val = __read_id_reg(vcpu, SYS_ID_AA64MMFR1_EL1);
> >         u32 sr = reg_to_encoding(r);
> >
> >         if (!(val & (0xfUL << ID_AA64MMFR1_LOR_SHIFT))) {
> > @@ -1059,17 +1061,9 @@ static bool access_arch_timer(struct kvm_vcpu *vcpu,
> >         return true;
> >  }
> >
> > -/* Read a sanitised cpufeature ID register by sys_reg_desc */
> > -static u64 read_id_reg(const struct kvm_vcpu *vcpu,
> > -               struct sys_reg_desc const *r, bool raz)
> > +static u64 __read_id_reg(const struct kvm_vcpu *vcpu, u32 id)
> >  {
> > -       u32 id = reg_to_encoding(r);
> > -       u64 val;
> > -
> > -       if (raz)
> > -               return 0;
> > -
> > -       val = read_sanitised_ftr_reg(id);
> > +       u64 val = vcpu->kvm->arch.id_regs[IDREG_IDX(id)];
> >
> >         switch (id) {
> >         case SYS_ID_AA64PFR0_EL1:
> > @@ -1119,6 +1113,14 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu,
> >         return val;
> >  }
> >
> > +static u64 read_id_reg(const struct kvm_vcpu *vcpu,
> > +                      struct sys_reg_desc const *r, bool raz)
> > +{
> > +       u32 id = reg_to_encoding(r);
> > +
> > +       return raz ? 0 : __read_id_reg(vcpu, id);
> > +}
> > +
> >  static unsigned int id_visibility(const struct kvm_vcpu *vcpu,
> >                                   const struct sys_reg_desc *r)
> >  {
> > @@ -1223,9 +1225,8 @@ static int set_id_aa64pfr0_el1(struct kvm_vcpu *vcpu,
> >  /*
> >   * cpufeature ID register user accessors
> >   *
> > - * For now, these registers are immutable for userspace, so no values
> > - * are stored, and for set_id_reg() we don't allow the effective value
> > - * to be changed.
> > + * For now, these registers are immutable for userspace, so for set_id_reg()
> > + * we don't allow the effective value to be changed.
> >   */
> >  static int __get_id_reg(const struct kvm_vcpu *vcpu,
> >                         const struct sys_reg_desc *rd, void __user *uaddr,
> > @@ -1237,7 +1238,7 @@ static int __get_id_reg(const struct kvm_vcpu *vcpu,
> >         return reg_to_user(uaddr, &val, id);
> >  }
> >
> > -static int __set_id_reg(const struct kvm_vcpu *vcpu,
> > +static int __set_id_reg(struct kvm_vcpu *vcpu,
> >                         const struct sys_reg_desc *rd, void __user *uaddr,
> >                         bool raz)
>
> Minor nit: why remove the const in this patch? This is required for a
> future patch but not for this one.

Thank you for catching this. I will fix this.

Regards,
Reiji

>
>
> >  {
> > @@ -1837,8 +1838,8 @@ static bool trap_dbgdidr(struct kvm_vcpu *vcpu,
> >         if (p->is_write) {
> >                 return ignore_write(vcpu, p);
> >         } else {
> > -               u64 dfr = read_sanitised_ftr_reg(SYS_ID_AA64DFR0_EL1);
> > -               u64 pfr = read_sanitised_ftr_reg(SYS_ID_AA64PFR0_EL1);
> > +               u64 dfr = __read_id_reg(vcpu, SYS_ID_AA64DFR0_EL1);
> > +               u64 pfr = __read_id_reg(vcpu, SYS_ID_AA64PFR0_EL1);
> >                 u32 el3 = !!cpuid_feature_extract_unsigned_field(pfr, ID_AA64PFR0_EL3_SHIFT);
> >
> >                 p->regval = ((((dfr >> ID_AA64DFR0_WRPS_SHIFT) & 0xf) << 28) |
> > @@ -2850,3 +2851,30 @@ void kvm_sys_reg_table_init(void)
> >         /* Clear all higher bits. */
> >         cache_levels &= (1 << (i*3))-1;
> >  }
> > +
> > +/*
> > + * Set the guest's ID registers that are defined in sys_reg_descs[]
> > + * with ID_SANITISED() to the host's sanitized value.
> > + */
> > +void set_default_id_regs(struct kvm *kvm)
> > +{
> > +       int i;
> > +       u32 id;
> > +       const struct sys_reg_desc *rd;
> > +       u64 val;
> > +
> > +       for (i = 0; i < ARRAY_SIZE(sys_reg_descs); i++) {
> > +               rd = &sys_reg_descs[i];
> > +               if (rd->access != access_id_reg)
> > +                       /* Not ID register, or hidden/reserved ID register */
> > +                       continue;
> > +
> > +               id = reg_to_encoding(rd);
> > +               if (WARN_ON_ONCE(!is_id_reg(id)))
> > +                       /* Shouldn't happen */
> > +                       continue;
> > +
> > +               val = read_sanitised_ftr_reg(id);
> > +               kvm->arch.id_regs[IDREG_IDX(id)] = val;
> > +       }
> > +}
> > --
> > 2.34.1.448.ga2b2bfdf31-goog
> >
> > _______________________________________________
> > kvmarm mailing list
> > kvmarm@lists.cs.columbia.edu
> > https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

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