linux-kernel - Re: [PATCH 12/13] RISC-V: KVM: Save trap CSRs in kvm_riscv_vcpu_enter

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Message-ID: <CAOnJCULYc-m_Jq-MxwGMbktmCvLPsMiA_tzxE2qYuoKgeR0BEw@mail.gmail.com>
Date: Fri, 18 Oct 2024 13:11:18 -0700
From: Atish Patra <atishp@...shpatra.org>
To: Anup Patel <apatel@...tanamicro.com>
Cc: Palmer Dabbelt <palmer@...belt.com>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Andrew Jones <ajones@...tanamicro.com>, Anup Patel <anup@...infault.org>, kvm@...r.kernel.org, 
	kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 12/13] RISC-V: KVM: Save trap CSRs in kvm_riscv_vcpu_enter_exit()

On Fri, Jul 19, 2024 at 9:10 AM Anup Patel <apatel@...tanamicro.com> wrote:
>
> Save trap CSRs in the kvm_riscv_vcpu_enter_exit() function instead of
> the kvm_arch_vcpu_ioctl_run() function so that HTVAL and HTINST CSRs
> are accessed in more optimized manner while running under some other
> hypervisor.
>
> Signed-off-by: Anup Patel <apatel@...tanamicro.com>
> ---
>  arch/riscv/kvm/vcpu.c | 34 +++++++++++++++++++++-------------
>  1 file changed, 21 insertions(+), 13 deletions(-)
>
> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> index fe849fb1aaab..854d98aa165e 100644
> --- a/arch/riscv/kvm/vcpu.c
> +++ b/arch/riscv/kvm/vcpu.c
> @@ -757,12 +757,21 @@ static __always_inline void kvm_riscv_vcpu_swap_in_host_state(struct kvm_vcpu *v
>   * This must be noinstr as instrumentation may make use of RCU, and this is not
>   * safe during the EQS.
>   */
> -static void noinstr kvm_riscv_vcpu_enter_exit(struct kvm_vcpu *vcpu)
> +static void noinstr kvm_riscv_vcpu_enter_exit(struct kvm_vcpu *vcpu,
> +                                             struct kvm_cpu_trap *trap)
>  {
>         void *nsh;
>         struct kvm_cpu_context *gcntx = &vcpu->arch.guest_context;
>         struct kvm_cpu_context *hcntx = &vcpu->arch.host_context;
>
> +       /*
> +        * We save trap CSRs (such as SEPC, SCAUSE, STVAL, HTVAL, and
> +        * HTINST) here because we do local_irq_enable() after this
> +        * function in kvm_arch_vcpu_ioctl_run() which can result in
> +        * an interrupt immediately after local_irq_enable() and can
> +        * potentially change trap CSRs.
> +        */
> +
>         kvm_riscv_vcpu_swap_in_guest_state(vcpu);
>         guest_state_enter_irqoff();
>
> @@ -805,14 +814,24 @@ static void noinstr kvm_riscv_vcpu_enter_exit(struct kvm_vcpu *vcpu)
>                 } else {
>                         gcntx->hstatus = csr_swap(CSR_HSTATUS, hcntx->hstatus);
>                 }
> +
> +               trap->htval = nacl_csr_read(nsh, CSR_HTVAL);
> +               trap->htinst = nacl_csr_read(nsh, CSR_HTINST);
>         } else {
>                 hcntx->hstatus = csr_swap(CSR_HSTATUS, gcntx->hstatus);
>
>                 __kvm_riscv_switch_to(&vcpu->arch);
>
>                 gcntx->hstatus = csr_swap(CSR_HSTATUS, hcntx->hstatus);
> +
> +               trap->htval = csr_read(CSR_HTVAL);
> +               trap->htinst = csr_read(CSR_HTINST);
>         }
>
> +       trap->sepc = gcntx->sepc;
> +       trap->scause = csr_read(CSR_SCAUSE);
> +       trap->stval = csr_read(CSR_STVAL);
> +
>         vcpu->arch.last_exit_cpu = vcpu->cpu;
>         guest_state_exit_irqoff();
>         kvm_riscv_vcpu_swap_in_host_state(vcpu);
> @@ -929,22 +948,11 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>
>                 guest_timing_enter_irqoff();
>
> -               kvm_riscv_vcpu_enter_exit(vcpu);
> +               kvm_riscv_vcpu_enter_exit(vcpu, &trap);
>
>                 vcpu->mode = OUTSIDE_GUEST_MODE;
>                 vcpu->stat.exits++;
>
> -               /*
> -                * Save SCAUSE, STVAL, HTVAL, and HTINST because we might
> -                * get an interrupt between __kvm_riscv_switch_to() and
> -                * local_irq_enable() which can potentially change CSRs.
> -                */
> -               trap.sepc = vcpu->arch.guest_context.sepc;
> -               trap.scause = csr_read(CSR_SCAUSE);
> -               trap.stval = csr_read(CSR_STVAL);
> -               trap.htval = ncsr_read(CSR_HTVAL);
> -               trap.htinst = ncsr_read(CSR_HTINST);
> -
>                 /* Syncup interrupts state with HW */
>                 kvm_riscv_vcpu_sync_interrupts(vcpu);
>
> --
> 2.34.1
>


Reviewed-by: Atish Patra <atishp@...osinc.com>
-- 
Regards,
Atish