[PATCH v6 6/8] RISC-V: KVM: No need of explicit writable slot check

[Anup Patel]

On Tue, Sep 9, 2025 at 12:33 PM Atish Patra <atishp at rivosinc.com> wrote:
>
> There is not much value in checking if a memslot is writable explicitly
> before a write as it may change underneath after the check. Rather, return
> invalid address error when write_guest fails as it checks if the slot
> is writable anyways.
>
> Suggested-by: Sean Christopherson <seanjc at google.com>
> Signed-off-by: Atish Patra <atishp at rivosinc.com>

LGTM.

Reviewed-by: Anup Patel <anup at brainfault.org>

Regards,
Anup

> ---
>  arch/riscv/kvm/vcpu_pmu.c     | 11 ++---------
>  arch/riscv/kvm/vcpu_sbi_sta.c |  9 ++-------
>  2 files changed, 4 insertions(+), 16 deletions(-)
>
> diff --git a/arch/riscv/kvm/vcpu_pmu.c b/arch/riscv/kvm/vcpu_pmu.c
> index 15d71a7b75ba..f8514086bd6b 100644
> --- a/arch/riscv/kvm/vcpu_pmu.c
> +++ b/arch/riscv/kvm/vcpu_pmu.c
> @@ -409,8 +409,6 @@ int kvm_riscv_vcpu_pmu_snapshot_set_shmem(struct kvm_vcpu *vcpu, unsigned long s
>         int snapshot_area_size = sizeof(struct riscv_pmu_snapshot_data);
>         int sbiret = 0;
>         gpa_t saddr;
> -       unsigned long hva;
> -       bool writable;
>
>         if (!kvpmu || flags) {
>                 sbiret = SBI_ERR_INVALID_PARAM;
> @@ -432,19 +430,14 @@ int kvm_riscv_vcpu_pmu_snapshot_set_shmem(struct kvm_vcpu *vcpu, unsigned long s
>                 goto out;
>         }
>
> -       hva = kvm_vcpu_gfn_to_hva_prot(vcpu, saddr >> PAGE_SHIFT, &writable);
> -       if (kvm_is_error_hva(hva) || !writable) {
> -               sbiret = SBI_ERR_INVALID_ADDRESS;
> -               goto out;
> -       }
> -
>         kvpmu->sdata = kzalloc(snapshot_area_size, GFP_ATOMIC);
>         if (!kvpmu->sdata)
>                 return -ENOMEM;
>
> +       /* No need to check writable slot explicitly as kvm_vcpu_write_guest does it internally */
>         if (kvm_vcpu_write_guest(vcpu, saddr, kvpmu->sdata, snapshot_area_size)) {
>                 kfree(kvpmu->sdata);
> -               sbiret = SBI_ERR_FAILURE;
> +               sbiret = SBI_ERR_INVALID_ADDRESS;
>                 goto out;
>         }
>
> diff --git a/arch/riscv/kvm/vcpu_sbi_sta.c b/arch/riscv/kvm/vcpu_sbi_sta.c
> index cc6cb7c8f0e4..caaa28460ca4 100644
> --- a/arch/riscv/kvm/vcpu_sbi_sta.c
> +++ b/arch/riscv/kvm/vcpu_sbi_sta.c
> @@ -85,8 +85,6 @@ static int kvm_sbi_sta_steal_time_set_shmem(struct kvm_vcpu *vcpu)
>         unsigned long shmem_phys_hi = cp->a1;
>         u32 flags = cp->a2;
>         struct sbi_sta_struct zero_sta = {0};
> -       unsigned long hva;
> -       bool writable;
>         gpa_t shmem;
>         int ret;
>
> @@ -111,13 +109,10 @@ static int kvm_sbi_sta_steal_time_set_shmem(struct kvm_vcpu *vcpu)
>                         return SBI_ERR_INVALID_ADDRESS;
>         }
>
> -       hva = kvm_vcpu_gfn_to_hva_prot(vcpu, shmem >> PAGE_SHIFT, &writable);
> -       if (kvm_is_error_hva(hva) || !writable)
> -               return SBI_ERR_INVALID_ADDRESS;
> -
> +       /* No need to check writable slot explicitly as kvm_vcpu_write_guest does it internally */
>         ret = kvm_vcpu_write_guest(vcpu, shmem, &zero_sta, sizeof(zero_sta));
>         if (ret)
> -               return SBI_ERR_FAILURE;
> +               return SBI_ERR_INVALID_ADDRESS;
>
>         vcpu->arch.sta.shmem = shmem;
>         vcpu->arch.sta.last_steal = current->sched_info.run_delay;
>
> --
> 2.43.0
>