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Subject: Re: [PATCH v3 4/4] RISC-V: KVM: Support sstc extension
From:   Jessica Clarke <jrtc27@jrtc27.com>
In-Reply-To: <20220426185245.281182-5-atishp@rivosinc.com>
Date:   Tue, 26 Apr 2022 22:10:47 +0100
Cc:     "linux-kernel@vger.kernel.org List" <linux-kernel@vger.kernel.org>,
        Atish Patra <atishp@atishpatra.org>,
        Anup Patel <anup@brainfault.org>,
        Damien Le Moal <damien.lemoal@wdc.com>,
        devicetree <devicetree@vger.kernel.org>,
        Jisheng Zhang <jszhang@kernel.org>,
        Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>,
        kvm@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Rob Herring <robh+dt@kernel.org>
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Message-Id: <19F90AE1-68AD-4478-B5C3-8ABADD781198@jrtc27.com>
References: <20220426185245.281182-1-atishp@rivosinc.com>
 <20220426185245.281182-5-atishp@rivosinc.com>
To:     Atish Patra <atishp@rivosinc.com>
Precedence: bulk

On 26 Apr 2022, at 19:52, Atish Patra <atishp@rivosinc.com> wrote:
>=20
> Sstc extension allows the guest to program the vstimecmp CSR directly
> instead of making an SBI call to the hypervisor to program the next
> event. The timer interrupt is also directly injected to the guest by
> the hardware in this case. To maintain backward compatibility, the
> hypervisors also update the vstimecmp in an SBI set_time call if
> the hardware supports it. Thus, the older kernels in guest also
> take advantage of the sstc extension.

This still violates the following part of the ratified SBI spec:

> =E2=80=A2 All registers except a0 & a1 must be preserved across an SBI =
call by the callee.

The Set Timer legacy extension and non-legacy function state they clear
the pending timer bit but otherwise make no provision for other S-mode
state being clobbered. The stimecmp register is S-mode read/write
state. I don=E2=80=99t debate that this is a useful thing to allow, but =
as
things stand this is in direct violation of the letter of the ratified
SBI spec and so if you want to allow this you have to fix your spec
first and deal with the ratified spec compatibility issues that brings.

Jess

> Signed-off-by: Atish Patra <atishp@rivosinc.com>
> ---
> arch/riscv/include/asm/kvm_host.h       |   1 +
> arch/riscv/include/asm/kvm_vcpu_timer.h |   8 +-
> arch/riscv/include/uapi/asm/kvm.h       |   1 +
> arch/riscv/kvm/main.c                   |  12 ++-
> arch/riscv/kvm/vcpu.c                   |   5 +-
> arch/riscv/kvm/vcpu_timer.c             | 138 +++++++++++++++++++++++-
> 6 files changed, 159 insertions(+), 6 deletions(-)
>=20
> diff --git a/arch/riscv/include/asm/kvm_host.h =
b/arch/riscv/include/asm/kvm_host.h
> index 78da839657e5..50a97c821f83 100644
> --- a/arch/riscv/include/asm/kvm_host.h
> +++ b/arch/riscv/include/asm/kvm_host.h
> @@ -135,6 +135,7 @@ struct kvm_vcpu_csr {
> 	unsigned long hvip;
> 	unsigned long vsatp;
> 	unsigned long scounteren;
> +	u64 vstimecmp;
> };
>=20
> struct kvm_vcpu_arch {
> diff --git a/arch/riscv/include/asm/kvm_vcpu_timer.h =
b/arch/riscv/include/asm/kvm_vcpu_timer.h
> index 375281eb49e0..a24a265f3ccb 100644
> --- a/arch/riscv/include/asm/kvm_vcpu_timer.h
> +++ b/arch/riscv/include/asm/kvm_vcpu_timer.h
> @@ -28,6 +28,11 @@ struct kvm_vcpu_timer {
> 	u64 next_cycles;
> 	/* Underlying hrtimer instance */
> 	struct hrtimer hrt;
> +
> +	/* Flag to check if sstc is enabled or not */
> +	bool sstc_enabled;
> +	/* A function pointer to switch between stimecmp or hrtimer at =
runtime */
> +	int (*timer_next_event)(struct kvm_vcpu *vcpu, u64 ncycles);
> };
>=20
> int kvm_riscv_vcpu_timer_next_event(struct kvm_vcpu *vcpu, u64 =
ncycles);
> @@ -39,6 +44,7 @@ int kvm_riscv_vcpu_timer_init(struct kvm_vcpu =
*vcpu);
> int kvm_riscv_vcpu_timer_deinit(struct kvm_vcpu *vcpu);
> int kvm_riscv_vcpu_timer_reset(struct kvm_vcpu *vcpu);
> void kvm_riscv_vcpu_timer_restore(struct kvm_vcpu *vcpu);
> +void kvm_riscv_vcpu_timer_save(struct kvm_vcpu *vcpu);
> int kvm_riscv_guest_timer_init(struct kvm *kvm);
> -
> +bool kvm_riscv_vcpu_timer_pending(struct kvm_vcpu *vcpu);
> #endif
> diff --git a/arch/riscv/include/uapi/asm/kvm.h =
b/arch/riscv/include/uapi/asm/kvm.h
> index 92bd469e2ba6..d2f02ba1947a 100644
> --- a/arch/riscv/include/uapi/asm/kvm.h
> +++ b/arch/riscv/include/uapi/asm/kvm.h
> @@ -96,6 +96,7 @@ enum KVM_RISCV_ISA_EXT_ID {
> 	KVM_RISCV_ISA_EXT_H,
> 	KVM_RISCV_ISA_EXT_I,
> 	KVM_RISCV_ISA_EXT_M,
> +	KVM_RISCV_ISA_EXT_SSTC,
> 	KVM_RISCV_ISA_EXT_MAX,
> };
>=20
> diff --git a/arch/riscv/kvm/main.c b/arch/riscv/kvm/main.c
> index 2e5ca43c8c49..83c4db7fc35f 100644
> --- a/arch/riscv/kvm/main.c
> +++ b/arch/riscv/kvm/main.c
> @@ -32,7 +32,7 @@ int kvm_arch_hardware_setup(void *opaque)
>=20
> int kvm_arch_hardware_enable(void)
> {
> -	unsigned long hideleg, hedeleg;
> +	unsigned long hideleg, hedeleg, henvcfg;
>=20
> 	hedeleg =3D 0;
> 	hedeleg |=3D (1UL << EXC_INST_MISALIGNED);
> @@ -51,6 +51,16 @@ int kvm_arch_hardware_enable(void)
>=20
> 	csr_write(CSR_HCOUNTEREN, -1UL);
>=20
> +	if (riscv_isa_extension_available(NULL, SSTC)) {
> +#ifdef CONFIG_64BIT
> +		henvcfg =3D csr_read(CSR_HENVCFG);
> +		csr_write(CSR_HENVCFG, henvcfg | 1UL<<HENVCFG_STCE);
> +#else
> +		henvcfg =3D csr_read(CSR_HENVCFGH);
> +		csr_write(CSR_HENVCFGH, henvcfg | 1UL<<HENVCFGH_STCE);
> +#endif
> +	}
> +
> 	csr_write(CSR_HVIP, 0);
>=20
> 	return 0;
> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> index 93492eb292fd..da1559725b03 100644
> --- a/arch/riscv/kvm/vcpu.c
> +++ b/arch/riscv/kvm/vcpu.c
> @@ -143,7 +143,7 @@ void kvm_arch_vcpu_destroy(struct kvm_vcpu *vcpu)
>=20
> int kvm_cpu_has_pending_timer(struct kvm_vcpu *vcpu)
> {
> -	return kvm_riscv_vcpu_has_interrupts(vcpu, 1UL << IRQ_VS_TIMER);
> +	return kvm_riscv_vcpu_timer_pending(vcpu);
> }
>=20
> void kvm_arch_vcpu_blocking(struct kvm_vcpu *vcpu)
> @@ -374,6 +374,7 @@ static unsigned long kvm_isa_ext_arr[] =3D {
> 	RISCV_ISA_EXT_h,
> 	RISCV_ISA_EXT_i,
> 	RISCV_ISA_EXT_m,
> +	RISCV_ISA_EXT_SSTC,
> };
>=20
> static int kvm_riscv_vcpu_get_reg_isa_ext(struct kvm_vcpu *vcpu,
> @@ -754,6 +755,8 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> 				     vcpu->arch.isa);
> 	kvm_riscv_vcpu_host_fp_restore(&vcpu->arch.host_context);
>=20
> +	kvm_riscv_vcpu_timer_save(vcpu);
> +
> 	csr->vsstatus =3D csr_read(CSR_VSSTATUS);
> 	csr->vsie =3D csr_read(CSR_VSIE);
> 	csr->vstvec =3D csr_read(CSR_VSTVEC);
> diff --git a/arch/riscv/kvm/vcpu_timer.c b/arch/riscv/kvm/vcpu_timer.c
> index 5c4c37ff2d48..d226a931de92 100644
> --- a/arch/riscv/kvm/vcpu_timer.c
> +++ b/arch/riscv/kvm/vcpu_timer.c
> @@ -69,7 +69,18 @@ static int kvm_riscv_vcpu_timer_cancel(struct =
kvm_vcpu_timer *t)
> 	return 0;
> }
>=20
> -int kvm_riscv_vcpu_timer_next_event(struct kvm_vcpu *vcpu, u64 =
ncycles)
> +static int kvm_riscv_vcpu_update_vstimecmp(struct kvm_vcpu *vcpu, u64 =
ncycles)
> +{
> +#if __riscv_xlen =3D=3D 32
> +		csr_write(CSR_VSTIMECMP, ncycles & 0xFFFFFFFF);
> +		csr_write(CSR_VSTIMECMPH, ncycles >> 32);
> +#else
> +		csr_write(CSR_VSTIMECMP, ncycles);
> +#endif
> +		return 0;
> +}
> +
> +static int kvm_riscv_vcpu_update_hrtimer(struct kvm_vcpu *vcpu, u64 =
ncycles)
> {
> 	struct kvm_vcpu_timer *t =3D &vcpu->arch.timer;
> 	struct kvm_guest_timer *gt =3D &vcpu->kvm->arch.timer;
> @@ -88,6 +99,68 @@ int kvm_riscv_vcpu_timer_next_event(struct kvm_vcpu =
*vcpu, u64 ncycles)
> 	return 0;
> }
>=20
> +int kvm_riscv_vcpu_timer_next_event(struct kvm_vcpu *vcpu, u64 =
ncycles)
> +{
> +	struct kvm_vcpu_timer *t =3D &vcpu->arch.timer;
> +
> +	return t->timer_next_event(vcpu, ncycles);
> +}
> +
> +static enum hrtimer_restart kvm_riscv_vcpu_vstimer_expired(struct =
hrtimer *h)
> +{
> +	u64 delta_ns;
> +	struct kvm_vcpu_timer *t =3D container_of(h, struct =
kvm_vcpu_timer, hrt);
> +	struct kvm_vcpu *vcpu =3D container_of(t, struct kvm_vcpu, =
arch.timer);
> +	struct kvm_guest_timer *gt =3D &vcpu->kvm->arch.timer;
> +
> +	if (kvm_riscv_current_cycles(gt) < t->next_cycles) {
> +		delta_ns =3D kvm_riscv_delta_cycles2ns(t->next_cycles, =
gt, t);
> +		hrtimer_forward_now(&t->hrt, ktime_set(0, delta_ns));
> +		return HRTIMER_RESTART;
> +	}
> +
> +	t->next_set =3D false;
> +	kvm_vcpu_kick(vcpu);
> +
> +	return HRTIMER_NORESTART;
> +}
> +
> +bool kvm_riscv_vcpu_timer_pending(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_vcpu_timer *t =3D &vcpu->arch.timer;
> +	struct kvm_guest_timer *gt =3D &vcpu->kvm->arch.timer;
> +	u64 vstimecmp_val =3D vcpu->arch.guest_csr.vstimecmp;
> +
> +	if (!kvm_riscv_delta_cycles2ns(vstimecmp_val, gt, t) ||
> +	    kvm_riscv_vcpu_has_interrupts(vcpu, 1UL << IRQ_VS_TIMER))
> +		return true;
> +	else
> +		return false;
> +}
> +
> +static void kvm_riscv_vcpu_timer_blocking(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_vcpu_timer *t =3D &vcpu->arch.timer;
> +	struct kvm_guest_timer *gt =3D &vcpu->kvm->arch.timer;
> +	u64 delta_ns;
> +	u64 vstimecmp_val =3D vcpu->arch.guest_csr.vstimecmp;
> +
> +	if (!t->init_done)
> +		return;
> +
> +	delta_ns =3D kvm_riscv_delta_cycles2ns(vstimecmp_val, gt, t);
> +	if (delta_ns) {
> +		t->next_cycles =3D vstimecmp_val;
> +		hrtimer_start(&t->hrt, ktime_set(0, delta_ns), =
HRTIMER_MODE_REL);
> +		t->next_set =3D true;
> +	}
> +}
> +
> +static void kvm_riscv_vcpu_timer_unblocking(struct kvm_vcpu *vcpu)
> +{
> +	kvm_riscv_vcpu_timer_cancel(&vcpu->arch.timer);
> +}
> +
> int kvm_riscv_vcpu_get_reg_timer(struct kvm_vcpu *vcpu,
> 				 const struct kvm_one_reg *reg)
> {
> @@ -180,10 +253,20 @@ int kvm_riscv_vcpu_timer_init(struct kvm_vcpu =
*vcpu)
> 		return -EINVAL;
>=20
> 	hrtimer_init(&t->hrt, CLOCK_MONOTONIC, HRTIMER_MODE_REL);
> -	t->hrt.function =3D kvm_riscv_vcpu_hrtimer_expired;
> 	t->init_done =3D true;
> 	t->next_set =3D false;
>=20
> +	/* Enable sstc for every vcpu if available in hardware */
> +	if (riscv_isa_extension_available(NULL, SSTC)) {
> +		t->sstc_enabled =3D true;
> +		t->hrt.function =3D kvm_riscv_vcpu_vstimer_expired;
> +		t->timer_next_event =3D kvm_riscv_vcpu_update_vstimecmp;
> +	} else {
> +		t->sstc_enabled =3D false;
> +		t->hrt.function =3D kvm_riscv_vcpu_hrtimer_expired;
> +		t->timer_next_event =3D kvm_riscv_vcpu_update_hrtimer;
> +	}
> +
> 	return 0;
> }
>=20
> @@ -202,7 +285,7 @@ int kvm_riscv_vcpu_timer_reset(struct kvm_vcpu =
*vcpu)
> 	return kvm_riscv_vcpu_timer_cancel(&vcpu->arch.timer);
> }
>=20
> -void kvm_riscv_vcpu_timer_restore(struct kvm_vcpu *vcpu)
> +static void kvm_riscv_vcpu_update_timedelta(struct kvm_vcpu *vcpu)
> {
> 	struct kvm_guest_timer *gt =3D &vcpu->kvm->arch.timer;
>=20
> @@ -214,6 +297,55 @@ void kvm_riscv_vcpu_timer_restore(struct kvm_vcpu =
*vcpu)
> #endif
> }
>=20
> +void kvm_riscv_vcpu_timer_restore(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_vcpu_csr *csr;
> +	struct kvm_vcpu_timer *t =3D &vcpu->arch.timer;
> +
> +	kvm_riscv_vcpu_update_timedelta(vcpu);
> +
> +	if (!t->sstc_enabled)
> +		return;
> +
> +	csr =3D &vcpu->arch.guest_csr;
> +#ifdef CONFIG_64BIT
> +	csr_write(CSR_VSTIMECMP, csr->vstimecmp);
> +#else
> +	csr_write(CSR_VSTIMECMP, (u32)csr->vstimecmp);
> +	csr_write(CSR_VSTIMECMPH, (u32)(csr->vstimecmp >> 32));
> +#endif
> +
> +	/* timer should be enabled for the remaining operations */
> +	if (unlikely(!t->init_done))
> +		return;
> +
> +	kvm_riscv_vcpu_timer_unblocking(vcpu);
> +}
> +
> +void kvm_riscv_vcpu_timer_save(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_vcpu_csr *csr;
> +	struct kvm_vcpu_timer *t =3D &vcpu->arch.timer;
> +
> +	if (!t->sstc_enabled)
> +		return;
> +
> +	csr =3D &vcpu->arch.guest_csr;
> +	t =3D &vcpu->arch.timer;
> +#ifdef CONFIG_64BIT
> +	csr->vstimecmp =3D csr_read(CSR_VSTIMECMP);
> +#else
> +	csr->vstimecmp =3D csr_read(CSR_VSTIMECMP);
> +	csr->vstimecmp |=3D (u64)csr_read(CSR_VSTIMECMPH) << 32;
> +#endif
> +	/* timer should be enabled for the remaining operations */
> +	if (unlikely(!t->init_done))
> +		return;
> +
> +	if (kvm_vcpu_is_blocking(vcpu))
> +		kvm_riscv_vcpu_timer_blocking(vcpu);
> +}
> +
> int kvm_riscv_guest_timer_init(struct kvm *kvm)
> {
> 	struct kvm_guest_timer *gt =3D &kvm->arch.timer;
> --=20
> 2.25.1
>=20
>=20
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