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@ventanamicro.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@vger.kernel.org Signed-off-by: Anup Patel apatel@ventanamicro.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