Re: [PATCH v2] RISC-V: KVM: Fix guest page fault within HLV* instructions

[Andrew Jones <ajones@ventanamicro.com>]

On Tue, Nov 11, 2025 at 09:55:06PM +0800, fangyu.yu@linux.alibaba.com wrote:
> From: Fangyu Yu <fangyu.yu@linux.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: b91f0e4cb8a3 ("RISC-V: KVM: Factor-out instruction emulation into separate sources")
> Signed-off-by: Fangyu Yu <fangyu.yu@linux.alibaba.com>
> 
> ---
> Changes in v2:
> - Remove unnecessary modifications and add comments(suggested by Anup)
> - Update Fixes tag
> - Link to v1: https://lore.kernel.org/linux-riscv/20250912134332.22053-1-fangyu.yu@linux.alibaba.com/
> ---
>  arch/riscv/kvm/vcpu_insn.c | 39 ++++++++++++++++++++++++++++++++++++++
>  1 file changed, 39 insertions(+)
> 
> diff --git a/arch/riscv/kvm/vcpu_insn.c b/arch/riscv/kvm/vcpu_insn.c
> index de1f96ea6225..a8d796ef2822 100644
> --- a/arch/riscv/kvm/vcpu_insn.c
> +++ b/arch/riscv/kvm/vcpu_insn.c
> @@ -323,6 +323,19 @@ int kvm_riscv_vcpu_virtual_insn(struct kvm_vcpu *vcpu, struct kvm_run *run,
>  							  ct->sepc,
>  							  &utrap);
>  			if (utrap.scause) {
> +				/**
> +				 * If a g-stage page fault occurs, the direct approach
> +				 * is to let the g-stage page fault handler handle it
> +				 * naturally, however, calling the g-stage page fault
> +				 * handler here seems rather strange.
> +				 * Considering this is an corner case, we can directly
> +				 * return to the guest and re-execute the same PC, this
> +				 * will trigger a g-stage page fault again and then the
> +				 * regular g-stage page fault handler will populate
> +				 * g-stage page table.
> +				 */
> +				if (utrap.scause == EXC_LOAD_GUEST_PAGE_FAULT)
> +					return 1;
>  				utrap.sepc = ct->sepc;
>  				kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
>  				return 1;
> @@ -378,6 +391,19 @@ 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) {
> +			/**
> +			 * If a g-stage page fault occurs, the direct approach
> +			 * is to let the g-stage page fault handler handle it
> +			 * naturally, however, calling the g-stage page fault
> +			 * handler here seems rather strange.
> +			 * Considering this is an corner case, we can directly
> +			 * return to the guest and re-execute the same PC, this
> +			 * will trigger a g-stage page fault again and then the
> +			 * regular g-stage page fault handler will populate
> +			 * g-stage page table.
> +			 */
> +			if (utrap.scause == EXC_LOAD_GUEST_PAGE_FAULT)
> +				return 1;
>  			/* Redirect trap if we failed to read instruction */
>  			utrap.sepc = ct->sepc;
>  			kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
> @@ -504,6 +530,19 @@ 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) {
> +			/**
> +			 * If a g-stage page fault occurs, the direct approach
> +			 * is to let the g-stage page fault handler handle it
> +			 * naturally, however, calling the g-stage page fault
> +			 * handler here seems rather strange.
> +			 * Considering this is an corner case, we can directly
> +			 * return to the guest and re-execute the same PC, this
> +			 * will trigger a g-stage page fault again and then the
> +			 * regular g-stage page fault handler will populate
> +			 * g-stage page table.
> +			 */
> +			if (utrap.scause == EXC_LOAD_GUEST_PAGE_FAULT)
> +				return 1;
>  			/* Redirect trap if we failed to read instruction */
>  			utrap.sepc = ct->sepc;
>  			kvm_riscv_vcpu_trap_redirect(vcpu, &utrap);
> -- 
> 2.50.1
>

To avoid repeating the same paragraph three times I would create a
helper function, kvm_riscv_check_load_guest_page_fault(), with the
paragraph placed in that function along with the utrap.scause
exception type check.

Thanks,
drew

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