linux-kernel - Re: [PATCH] RISC-V: KVM: Don't clear hgatp CSR in kvm_arch_vcpu

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Date:   Mon, 4 Apr 2022 09:20:27 +0530
From:   Anup Patel <anup@...infault.org>
To:     Anup Patel <apatel@...tanamicro.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Atish Patra <atishp@...shpatra.org>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Alistair Francis <Alistair.Francis@....com>,
        KVM General <kvm@...r.kernel.org>,
        "open list:KERNEL VIRTUAL MACHINE FOR RISC-V (KVM/riscv)" 
        <kvm-riscv@...ts.infradead.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org List" <linux-kernel@...r.kernel.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] RISC-V: KVM: Don't clear hgatp CSR in kvm_arch_vcpu_put()

On Thu, Mar 17, 2022 at 9:25 AM Anup Patel <apatel@...tanamicro.com> wrote:
>
> We might have RISC-V systems (such as QEMU) where VMID is not part
> of the TLB entry tag so these systems will have to flush all TLB
> entries upon any change in hgatp.VMID.
>
> Currently, we zero-out hgatp CSR in kvm_arch_vcpu_put() and we
> re-program hgatp CSR in kvm_arch_vcpu_load(). For above described
> systems, this will flush all TLB entries whenever VCPU exits to
> user-space hence reducing performance.
>
> This patch fixes above described performance issue by not clearing
> hgatp CSR in kvm_arch_vcpu_put().
>
> Fixes: 34bde9d8b9e6 ("RISC-V: KVM: Implement VCPU world-switch")
> Cc: stable@...r.kernel.org
> Signed-off-by: Anup Patel <apatel@...tanamicro.com>

I have queued this patch for RC fixes.

Thanks,
Anup

> ---
>  arch/riscv/kvm/vcpu.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
> index 624166004e36..6785aef4cbd4 100644
> --- a/arch/riscv/kvm/vcpu.c
> +++ b/arch/riscv/kvm/vcpu.c
> @@ -653,8 +653,6 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>                                      vcpu->arch.isa);
>         kvm_riscv_vcpu_host_fp_restore(&vcpu->arch.host_context);
>
> -       csr_write(CSR_HGATP, 0);
> -
>         csr->vsstatus = csr_read(CSR_VSSTATUS);
>         csr->vsie = csr_read(CSR_VSIE);
>         csr->vstvec = csr_read(CSR_VSTVEC);
> --
> 2.25.1
>