Re: [RFC PATCH 09/16] KVM: arm64: Allow ID registers to by dynamically read-as-zero

Christoffer Dall
2018-08-06T13:03:29Z

Hi Dave,

I think there's a typo in the subject "to be" rather than "to by".

On Thu, Jun 21, 2018 at 03:57:33PM +0100, Dave Martin wrote:
> When a feature-dependent ID register is hidden from the guest, it
> needs to exhibit read-as-zero behaviour as defined by the Arm
> architecture, rather than appearing to be entirely absent.
>
> This patch updates the ID register emulation logic to make use of
> the new check_present() method to determine whether the register
> should read as zero instead of yielding the host's sanitised
> value. Because currently a false result from this method truncates
> the trap call chain before the sysreg's emulate method() is called,
> a flag is added to distinguish this special case, and helpers are
> refactored appropriately.

I don't understand this last sentence.

And I'm not really sure I understand the code either.

I can't seem to see any registers which are defined as !present && !raz,
which is what I thought this feature was all about.

In other words, what is the benefit of this more generic method as
opposed to having a wrapper around read_id_reg() for read_sve_id_reg()
which sets RAZ if there is no support for SVE in this context?

Thanks,
-Christoffer

>
> This invloves some trivial updates to pass the vcpu pointer down
> into the ID register emulation/access functions.
>
> A new ID_SANITISED_IF() macro is defined for declaring
> conditionally visible ID registers.
>
> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> ---
> arch/arm64/kvm/sys_regs.c | 51 ++++++++++++++++++++++++++++++-----------------
> arch/arm64/kvm/sys_regs.h | 11 ++++++++++
> 2 files changed, 44 insertions(+), 18 deletions(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 31a351a..87d2468 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -987,11 +987,17 @@ static bool access_cntp_cval(struct kvm_vcpu *vcpu,
> }
>
> /* Read a sanitised cpufeature ID register by sys_reg_desc */
> -static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
> +static u64 read_id_reg(const struct kvm_vcpu *vcpu,
> + struct sys_reg_desc const *r, bool raz)
> {
> u32 id = sys_reg((u32)r->Op0, (u32)r->Op1,
> (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
> - u64 val = raz ? 0 : read_sanitised_ftr_reg(id);
> + u64 val;
> +
> + if (raz || !sys_reg_present(vcpu, r))
> + val = 0;
> + else
> + val = read_sanitised_ftr_reg(id);
>
> if (id == SYS_ID_AA64PFR0_EL1) {
> if (val & (0xfUL << ID_AA64PFR0_SVE_SHIFT))
> @@ -1018,7 +1024,7 @@ static bool __access_id_reg(struct kvm_vcpu *vcpu,
> if (p->is_write)
> return write_to_read_only(vcpu, p, r);
>
> - p->regval = read_id_reg(r, raz);
> + p->regval = read_id_reg(vcpu, r, raz);
> return true;
> }
>
> @@ -1047,16 +1053,18 @@ static u64 sys_reg_to_index(const struct sys_reg_desc *reg);
> * are stored, and for set_id_reg() we don't allow the effective value
> * to be changed.
> */
> -static int __get_id_reg(const struct sys_reg_desc *rd, void __user *uaddr,
> +static int __get_id_reg(const struct kvm_vcpu *vcpu,
> + const struct sys_reg_desc *rd, void __user *uaddr,
> bool raz)
> {
> const u64 id = sys_reg_to_index(rd);
> - const u64 val = read_id_reg(rd, raz);
> + const u64 val = read_id_reg(vcpu, rd, raz);
>
> return reg_to_user(uaddr, &val, id);
> }
>
> -static int __set_id_reg(const struct sys_reg_desc *rd, void __user *uaddr,
> +static int __set_id_reg(const struct kvm_vcpu *vcpu,
> + const struct sys_reg_desc *rd, void __user *uaddr,
> bool raz)
> {
> const u64 id = sys_reg_to_index(rd);
> @@ -1068,7 +1076,7 @@ static int __set_id_reg(const struct sys_reg_desc *rd, void __user *uaddr,
> return err;
>
> /* This is what we mean by invariant: you can't change it. */
> - if (val != read_id_reg(rd, raz))
> + if (val != read_id_reg(vcpu, rd, raz))
> return -EINVAL;
>
> return 0;
> @@ -1077,33 +1085,40 @@ static int __set_id_reg(const struct sys_reg_desc *rd, void __user *uaddr,
> static int get_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> const struct kvm_one_reg *reg, void __user *uaddr)
> {
> - return __get_id_reg(rd, uaddr, false);
> + return __get_id_reg(vcpu, rd, uaddr, false);
> }
>
> static int set_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> const struct kvm_one_reg *reg, void __user *uaddr)
> {
> - return __set_id_reg(rd, uaddr, false);
> + return __set_id_reg(vcpu, rd, uaddr, false);
> }
>
> static int get_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> const struct kvm_one_reg *reg, void __user *uaddr)
> {
> - return __get_id_reg(rd, uaddr, true);
> + return __get_id_reg(vcpu, rd, uaddr, true);
> }
>
> static int set_raz_id_reg(struct kvm_vcpu *vcpu, const struct sys_reg_desc *rd,
> const struct kvm_one_reg *reg, void __user *uaddr)
> {
> - return __set_id_reg(rd, uaddr, true);
> + return __set_id_reg(vcpu, rd, uaddr, true);
> }
>
> /* sys_reg_desc initialiser for known cpufeature ID registers */
> -#define ID_SANITISED(name) { \
> +#define __ID_SANITISED(name) \
> SYS_DESC(SYS_##name), \
> .access = access_id_reg, \
> .get_user = get_id_reg, \
> - .set_user = set_id_reg, \
> + .set_user = set_id_reg
> +
> +#define ID_SANITISED(name) { __ID_SANITISED(name) }
> +
> +#define ID_SANITISED_IF(name, check) { \
> + __ID_SANITISED(name), \
> + .check_present = check, \
> + .flags = SR_RAZ_IF_ABSENT, \
> }
>
> /*
> @@ -1840,7 +1855,7 @@ static int emulate_cp(struct kvm_vcpu *vcpu,
>
> r = find_reg(params, table, num);
>
> - if (likely(r) && sys_reg_present(vcpu, r)) {
> + if (likely(r) && sys_reg_present_or_raz(vcpu, r)) {
> perform_access(vcpu, params, r);
> return 0;
> }
> @@ -2016,7 +2031,7 @@ static int emulate_sys_reg(struct kvm_vcpu *vcpu,
> if (!r)
> r = find_reg(params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));
>
> - if (likely(r) && sys_reg_present(vcpu, r)) {
> + if (likely(r) && sys_reg_present_or_raz(vcpu, r)) {
> perform_access(vcpu, params, r);
> } else {
> kvm_err("Unsupported guest sys_reg access at: %lx\n",
> @@ -2313,7 +2328,7 @@ int kvm_arm_sys_reg_get_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
> if (!r)
> return get_invariant_sys_reg(reg->id, uaddr);
>
> - if (!sys_reg_present(vcpu, r))
> + if (!sys_reg_present_or_raz(vcpu, r))
> return -ENOENT;
>
> if (r->get_user)
> @@ -2337,7 +2352,7 @@ int kvm_arm_sys_reg_set_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg
> if (!r)
> return set_invariant_sys_reg(reg->id, uaddr);
>
> - if (!sys_reg_present(vcpu, r))
> + if (!sys_reg_present_or_raz(vcpu, r))
> return -ENOENT;
>
> if (r->set_user)
> @@ -2408,7 +2423,7 @@ static int walk_one_sys_reg(struct kvm_vcpu *vcpu,
> if (!(rd->reg || rd->get_user))
> return 0;
>
> - if (!sys_reg_present(vcpu, rd))
> + if (!sys_reg_present_or_raz(vcpu, rd))
> return 0;
>
> if (!copy_reg_to_user(rd, uind))
> diff --git a/arch/arm64/kvm/sys_regs.h b/arch/arm64/kvm/sys_regs.h
> index dfbb342..304928f 100644
> --- a/arch/arm64/kvm/sys_regs.h
> +++ b/arch/arm64/kvm/sys_regs.h
> @@ -66,14 +66,25 @@ struct sys_reg_desc {
> const struct kvm_one_reg *reg, void __user *uaddr);
> bool (*check_present)(const struct kvm_vcpu *vpcu,
> const struct sys_reg_desc *rd);
> +
> + /* OR of SR_* flags */
> + unsigned int flags;
> };
>
> +#define SR_RAZ_IF_ABSENT (1 << 0)
> +
> static inline bool sys_reg_present(const struct kvm_vcpu *vcpu,
> const struct sys_reg_desc *rd)
> {
> return likely(!rd->check_present) || rd->check_present(vcpu, rd);
> }
>
> +static inline bool sys_reg_present_or_raz(const struct kvm_vcpu *vcpu,
> + const struct sys_reg_desc *rd)
> +{
> + return sys_reg_present(vcpu, rd) || (rd->flags & SR_RAZ_IF_ABSENT);
> +}
> +
> static inline void print_sys_reg_instr(const struct sys_reg_params *p)
> {
> /* Look, we even formatted it for you to paste into the table! */
> --
> 2.1.4
>
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