Hi Jing,
On Mon, Apr 24, 2023 at 11:47:00PM +0000, Jing Zhang 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).
>
> No functional change intended.
>
> Co-developed-by: Reiji Watanabe <reijiw@xxxxxxxxxx>
> Signed-off-by: Reiji Watanabe <reijiw@xxxxxxxxxx>
> Signed-off-by: Jing Zhang <jingzhangos@xxxxxxxxxx>
> ---
> arch/arm64/include/asm/kvm_host.h | 47 +++++++++++++++++++++++++++++
> arch/arm64/kvm/arm.c | 1 +
> arch/arm64/kvm/id_regs.c | 49 +++++++++++++++++++++++++------
> arch/arm64/kvm/sys_regs.c | 2 +-
> arch/arm64/kvm/sys_regs.h | 3 +-
> 5 files changed, 90 insertions(+), 12 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index bcd774d74f34..2b1fe90a1790 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -177,6 +177,20 @@ struct kvm_smccc_features {
> unsigned long vendor_hyp_bmap;
> };
>
> +/*
> + * Emualted CPU ID registers per VM
typo: emulated
> + * (Op0, Op1, CRn, CRm, Op2) of the ID registers to be saved in it
> + * is (3, 0, 0, crm, op2), where 1<=crm<8, 0<=op2<8.
> + *
> + * These emulated idregs are VM-wide, but accessed from the context of a vCPU.
> + * Access to id regs are guarded by kvm_arch.config_lock.
> + */
> +#define KVM_ARM_ID_REG_NUM 56
> +#define IDREG_IDX(id) (((sys_reg_CRm(id) - 1) << 3) | sys_reg_Op2(id))
> +struct kvm_idregs {
> + u64 regs[KVM_ARM_ID_REG_NUM];
> +};
What is the purpose of declaring the register array as a separate
structure? It has no meaning (nor use) outside of the context of a VM.
I'd prefer the 'regs' array be embedded directly in kvm_arch, and just
name it 'idregs'. You can move your macro definitions there as well to
immediately precede the field.
> typedef unsigned int pkvm_handle_t;
>
> struct kvm_protected_vm {
> @@ -243,6 +257,9 @@ struct kvm_arch {
> /* Hypercall features firmware registers' descriptor */
> struct kvm_smccc_features smccc_feat;
>
> + /* Emulated CPU ID registers */
> + struct kvm_idregs idregs;
> +
> /*
> * For an untrusted host VM, 'pkvm.handle' is used to lookup
> * the associated pKVM instance in the hypervisor.
> @@ -1008,6 +1025,8 @@ 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 kvm_arm_init_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);
> @@ -1073,4 +1092,32 @@ static inline void kvm_hyp_reserve(void) { }
> void kvm_arm_vcpu_power_off(struct kvm_vcpu *vcpu);
> bool kvm_arm_vcpu_stopped(struct kvm_vcpu *vcpu);
>
> +static inline u64 _idreg_read(struct kvm_arch *arch, u32 id)
<bikeshed>
Personally, I find passing 'kvm_arch' around to be a bit clunky. Almost
all functions in KVM take 'struct kvm' as an argument, even if it only
accesses the data in 'kvm_arch'.
So, I'd prefer if all these helpers took 'struct kvm *'.
</bikeshed>
> +{
> + return arch->idregs.regs[IDREG_IDX(id)];
> +}
> +
> +static inline void _idreg_write(struct kvm_arch *arch, u32 id, u64 val)
> +{
> + arch->idregs.regs[IDREG_IDX(id)] = val;
> +}
> +
> +static inline u64 idreg_read(struct kvm_arch *arch, u32 id)
> +{
> + u64 val;
> +
> + mutex_lock(&arch->config_lock);
> + val = _idreg_read(arch, id);
> + mutex_unlock(&arch->config_lock);
What exactly are we protecting against by taking the config_lock here?
While I do believe there is value in serializing writers to the shared
data, there isn't a need to serialize reads from the guest.
What about implementing the following:
- Acquire the config_lock for handling writes. Only allow the value to
change if !kvm_vm_has_ran_once(). Otherwise, permit identical writes
(useful for hotplug, I imagine) or return EBUSY if userspace tried to
change something after running the VM.
- Acquire the config_lock for handling reads *from userspace*
- Handle reads from the guest with a plain old load, avoiding the need
to acquire any locks.
This has the benefit of avoiding lock contention for guest reads w/o
requiring the use of atomic loads/stores (i.e. {READ,WRITE}_ONCE()) to
protect said readers.
> + return val;
> +}
> +
> +static inline void idreg_write(struct kvm_arch *arch, u32 id, u64 val)
> +{
> + mutex_lock(&arch->config_lock);
> + _idreg_write(arch, id, val);
> + mutex_unlock(&arch->config_lock);
> +}
> +
> #endif /* __ARM64_KVM_HOST_H__ */
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 4b2e16e696a8..e34744c36406 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -153,6 +153,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>
> set_default_spectre(kvm);
> kvm_arm_init_hypercalls(kvm);
> + kvm_arm_init_id_regs(kvm);
>
> /*
> * Initialise the default PMUver before there is a chance to
> diff --git a/arch/arm64/kvm/id_regs.c b/arch/arm64/kvm/id_regs.c
> index 96b4c43a5100..d2fba2fde01c 100644
> --- a/arch/arm64/kvm/id_regs.c
> +++ b/arch/arm64/kvm/id_regs.c
> @@ -52,16 +52,9 @@ static u8 pmuver_to_perfmon(u8 pmuver)
> }
> }
>
> -/* 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)
> +u64 kvm_arm_read_id_reg(const struct kvm_vcpu *vcpu, u32 id)
> {
> - u32 id = reg_to_encoding(r);
> - u64 val;
> -
> - if (sysreg_visible_as_raz(vcpu, r))
> - return 0;
> -
> - val = read_sanitised_ftr_reg(id);
> + u64 val = idreg_read(&vcpu->kvm->arch, id);
>
> switch (id) {
> case SYS_ID_AA64PFR0_EL1:
> @@ -126,6 +119,14 @@ static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r
> return val;
> }
>
> +static u64 read_id_reg(const struct kvm_vcpu *vcpu, struct sys_reg_desc const *r)
> +{
> + if (sysreg_visible_as_raz(vcpu, r))
> + return 0;
> +
> + return kvm_arm_read_id_reg(vcpu, reg_to_encoding(r));
> +}
> +
> /* cpufeature ID register access trap handlers */
>
> static bool access_id_reg(struct kvm_vcpu *vcpu,
> @@ -458,3 +459,33 @@ int emulate_id_reg(struct kvm_vcpu *vcpu, struct sys_reg_params *params)
>
> return 1;
> }
> +
> +/*
> + * Set the guest's ID registers that are defined in id_reg_descs[]
> + * with ID_SANITISED() to the host's sanitized value.
> + */
> +void kvm_arm_init_id_regs(struct kvm *kvm)
> +{
> + int i;
> + u32 id;
> + u64 val;
nit: use reverse christmas/fir tree ordering for locals.
> + for (i = 0; i < ARRAY_SIZE(id_reg_descs); i++) {
> + id = reg_to_encoding(&id_reg_descs[i]);
> + if (WARN_ON_ONCE(!is_id_reg(id)))
> + /* Shouldn't happen */
> + continue;
I'll make the suggestion once more.
Please do not implement these sort of sanity checks on static data
structures at the point userspace has gotten involved. Sanity checking
on id_reg_descs[] should happen at the time KVM is initialized. If
anything is wrong at that point we should return an error and outright
refuse to run KVM.
--
Thanks,
Oliver
