Subject: Re: [PATCH v3 19/27] KVM: arm64: nv: Add fine grained trap forwarding infrastructure

From: Marc Zyngier <[email-masked]>
Date: Tue, 15 Aug 2023 11:39:34 +0100

On Mon, 14 Aug 2023 18:18:57 +0100,
Jing Zhang <[email-masked]> wrote:
> 
> Hi Marc,
> 
> On Tue, Aug 8, 2023 at 4:47 AM Marc Zyngier <[email-masked]> wrote:
> >
> > Fine Grained Traps are fun. Not.
> >
> > Implement the fine grained trap forwarding, reusing the Coarse Grained
> > Traps infrastructure previously implemented.
> >
> > Each sysreg/instruction inserted in the xarray gets a FGT group
> > (vaguely equivalent to a register number), a bit number in that register,
> > and a polarity.
> >
> > It is then pretty easy to check the FGT state at handling time, just
> > like we do for the coarse version (it is just faster).
> >
> > Reviewed-by: Eric Auger <[email-masked]>
> > Signed-off-by: Marc Zyngier <[email-masked]>
> > ---
> >  arch/arm64/kvm/emulate-nested.c | 78 ++++++++++++++++++++++++++++++++-
> >  1 file changed, 77 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kvm/emulate-nested.c b/arch/arm64/kvm/emulate-nested.c
> > index cd0544c3577e..af75c2775638 100644
> > --- a/arch/arm64/kvm/emulate-nested.c
> > +++ b/arch/arm64/kvm/emulate-nested.c
> > @@ -928,6 +928,27 @@ static const struct encoding_to_trap_config encoding_to_cgt[] __initconst = {
> >
> >  static DEFINE_XARRAY(sr_forward_xa);
> >
> > +enum fgt_group_id {
> > +       __NO_FGT_GROUP__,
> > +
> > +       /* Must be last */
> > +       __NR_FGT_GROUP_IDS__
> > +};
> > +
> > +#define SR_FGT(sr, g, b, p)                                    \
> > +       {                                                       \
> > +               .encoding       = sr,                           \
> > +               .end            = sr,                           \
> > +               .tc             = {                             \
> > +                       .fgt = g ## _GROUP,                     \
> > +                       .bit = g ## _EL2_ ## b ## _SHIFT,       \
> > +                       .pol = p,                               \
> > +               },                                              \
> > +       }
> > +
> > +static const struct encoding_to_trap_config encoding_to_fgt[] __initconst = {
> > +};
> > +
> >  static union trap_config get_trap_config(u32 sysreg)
> >  {
> >         return (union trap_config) {
> > @@ -941,6 +962,7 @@ int __init populate_nv_trap_config(void)
> >
> >         BUILD_BUG_ON(sizeof(union trap_config) != sizeof(void *));
> >         BUILD_BUG_ON(__NR_TRAP_GROUP_IDS__ > BIT(TC_CGT_BITS));
> > +       BUILD_BUG_ON(__NR_FGT_GROUP_IDS__ > BIT(TC_FGT_BITS));
> >
> >         for (int i = 0; i < ARRAY_SIZE(encoding_to_cgt); i++) {
> >                 const struct encoding_to_trap_config *cgt = &encoding_to_cgt[i];
> > @@ -963,6 +985,34 @@ int __init populate_nv_trap_config(void)
> >         kvm_info("nv: %ld coarse grained trap handlers\n",
> >                  ARRAY_SIZE(encoding_to_cgt));
> >
> > +       if (!cpus_have_final_cap(ARM64_HAS_FGT))
> > +               goto check_mcb;
> > +
> > +       for (int i = 0; i < ARRAY_SIZE(encoding_to_fgt); i++) {
> > +               const struct encoding_to_trap_config *fgt = &encoding_to_fgt[i];
> > +               union trap_config tc;
> > +
> > +               tc = get_trap_config(fgt->encoding);
> > +
> > +               if (tc.fgt) {
> > +                       kvm_err("Duplicate FGT for (%d, %d, %d, %d, %d)\n",
> > +                               sys_reg_Op0(fgt->encoding),
> > +                               sys_reg_Op1(fgt->encoding),
> > +                               sys_reg_CRn(fgt->encoding),
> > +                               sys_reg_CRm(fgt->encoding),
> > +                               sys_reg_Op2(fgt->encoding));
> > +                       ret = -EINVAL;
> > +               }
> > +
> > +               tc.val |= fgt->tc.val;
> > +               xa_store(&sr_forward_xa, fgt->encoding,
> > +                        xa_mk_value(tc.val), GFP_KERNEL);
> > +       }
> > +
> > +       kvm_info("nv: %ld fine grained trap handlers\n",
> > +                ARRAY_SIZE(encoding_to_fgt));
> > +
> > +check_mcb:
> >         for (int id = __MULTIPLE_CONTROL_BITS__;
> >              id < (__COMPLEX_CONDITIONS__ - 1);
> >              id++) {
> > @@ -1031,13 +1081,26 @@ static enum trap_behaviour compute_trap_behaviour(struct kvm_vcpu *vcpu,
> >         return __do_compute_trap_behaviour(vcpu, tc.cgt, b);
> >  }
> >
> > +static bool check_fgt_bit(u64 val, const union trap_config tc)
> > +{
> > +       return ((val >> tc.bit) & 1) == tc.pol;
> > +}
> > +
> > +#define sanitised_sys_reg(vcpu, reg)                   \
> > +       ({                                              \
> > +               u64 __val;                              \
> > +               __val = __vcpu_sys_reg(vcpu, reg);      \
> > +               __val &= ~__ ## reg ## _RES0;           \
> > +               (__val);                                \
> > +       })
> > +
> >  bool __check_nv_sr_forward(struct kvm_vcpu *vcpu)
> >  {
> >         union trap_config tc;
> >         enum trap_behaviour b;
> >         bool is_read;
> >         u32 sysreg;
> > -       u64 esr;
> > +       u64 esr, val;
> >
> >         if (!vcpu_has_nv(vcpu) || is_hyp_ctxt(vcpu))
> >                 return false;
> > @@ -1060,6 +1123,19 @@ bool __check_nv_sr_forward(struct kvm_vcpu *vcpu)
> >         if (!tc.val)
> >                 return false;
> >
> > +       switch ((enum fgt_group_id)tc.fgt) {
> > +       case __NO_FGT_GROUP__:
> > +               break;
> > +
> > +       case __NR_FGT_GROUP_IDS__:
> > +               /* Something is really wrong, bail out */
> > +               WARN_ONCE(1, "__NR_FGT_GROUP_IDS__");
> > +               return false;
> 
> Do we need a default clause here to catch unexpected tc.fgt values?

I'd rather not have anything special at handling time, as the compiler
is perfectly allowed to use the cast above to restrict the value to
the enumeration. We already cover all the possible enum values, which
is good enough.

However, I've added an extra check at boot time for unexpected values
having sneaked into the FGT table.

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.

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