linux-kernel - Re: [PATCH] RISC-V: KVM: Fix guest page fault within HLV* instructions

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Message-ID: <CAAhSdy0azfC-L3buko7-mN1PDWxi08HN=3NQ+0VeyMR8gVJNoQ@mail.gmail.com>
Date: Thu, 6 Nov 2025 11:48:17 +0530
From: Anup Patel <anup@...infault.org>
To: fangyu.yu@...ux.alibaba.com
Cc: atish.patra@...ux.dev, paul.walmsley@...ive.com, palmer@...belt.com, 
	aou@...s.berkeley.edu, alex@...ti.fr, pbonzini@...hat.com, graf@...zon.com, 
	jiangyifei@...wei.com, guoren@...nel.org, kvm@...r.kernel.org, 
	kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] RISC-V: KVM: Fix guest page fault within HLV* instructions

On Fri, Sep 12, 2025 at 7:13 PM <fangyu.yu@...ux.alibaba.com> wrote:
>
> From: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
>
> When executing HLV* instructions at the HS mode, a guest page fault
> may occur when a g-stage page table migration between triggering the
> virtual instruction exception and executing the HLV* instruction.
>
> This may be a corner case, and one simpler way to handle this is to
> re-execute the instruction where the virtual  instruction exception
> occurred, and the guest page fault will be automatically handled.
>
> Fixes: 9f7013265112 ("RISC-V: KVM: Handle MMIO exits for VCPU")
> Signed-off-by: Fangyu Yu <fangyu.yu@...ux.alibaba.com>
> ---
>  arch/riscv/kvm/vcpu_insn.c | 21 ++++++++++++++++++---
>  1 file changed, 18 insertions(+), 3 deletions(-)
>
> diff --git a/arch/riscv/kvm/vcpu_insn.c b/arch/riscv/kvm/vcpu_insn.c
> index 97dec18e6989..a8b93aa4d8ec 100644
> --- a/arch/riscv/kvm/vcpu_insn.c
> +++ b/arch/riscv/kvm/vcpu_insn.c
> @@ -448,7 +448,12 @@ int kvm_riscv_vcpu_virtual_insn(struct kvm_vcpu *vcpu, struct kvm_run *run,
>                         insn = kvm_riscv_vcpu_unpriv_read(vcpu, true,
>                                                           ct->sepc,
>                                                           &utrap);
> -                       if (utrap.scause) {
> +                       switch (utrap.scause) {
> +                       case 0:
> +                               break;

This looks like an unrelated change so drop it or send a separate patch
with proper explanation.

> +                       case EXC_LOAD_GUEST_PAGE_FAULT:
> +                               return KVM_INSN_CONTINUE_SAME_SEPC;

The KVM_INSN_xyz enum values are only for functions called via
system_opcode_insn() so return 1 over here just like the below
default case.

Also, add some comments over here about why we are simply
continuing the guest.

> +                       default:
>                                 utrap.sepc = ct->sepc;
>                                 kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
>                                 return 1;
> @@ -503,7 +508,12 @@ int kvm_riscv_vcpu_mmio_load(struct kvm_vcpu *vcpu, struct kvm_run *run,
>                  */
>                 insn = kvm_riscv_vcpu_unpriv_read(vcpu, true, ct->sepc,
>                                                   &utrap);
> -               if (utrap.scause) {
> +               switch (utrap.scause) {
> +               case 0:
> +                       break;

This looks like an unrelated change so drop it or send a separate patch
with proper explanation.

> +               case EXC_LOAD_GUEST_PAGE_FAULT:
> +                       return KVM_INSN_CONTINUE_SAME_SEPC;

Same comment as kvm_riscv_vcpu_virtual_insn().

> +               default:
>                         /* Redirect trap if we failed to read instruction */
>                         utrap.sepc = ct->sepc;
>                         kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
> @@ -629,7 +639,12 @@ int kvm_riscv_vcpu_mmio_store(struct kvm_vcpu *vcpu, struct kvm_run *run,
>                  */
>                 insn = kvm_riscv_vcpu_unpriv_read(vcpu, true, ct->sepc,
>                                                   &utrap);
> -               if (utrap.scause) {
> +               switch (utrap.scause) {
> +               case 0:
> +                       break;

This looks like an unrelated change so drop it or send a separate patch
with proper explanation.

> +               case EXC_LOAD_GUEST_PAGE_FAULT:
> +                       return KVM_INSN_CONTINUE_SAME_SEPC;

Same comment as kvm_riscv_vcpu_virtual_insn().

> +               default:
>                         /* Redirect trap if we failed to read instruction */
>                         utrap.sepc = ct->sepc;
>                         kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
> --
> 2.49.0
>
>

Regards,
Anup