Re: [PATCH] RISC-V: KVM: Introduce KVM_EXIT_FAIL_ENTRY_NO_VSFILE

[Anup Patel <anup@brainfault.org>]

On Mon, Jul 14, 2025 at 12:51 PM BillXiang
<xiangwencheng@lanxincomputing.com> wrote:
>
> Consider a system with 8 harts, where each hart supports 5
> Guest Interrupt Files (GIFs), yielding 40 total GIFs.
> If we launch a QEMU guest with over 5 vCPUs using
> "-M virt,aia='aplic-imsic' -accel kvm,riscv-aia=hwaccel" – which
> relies solely on VS-files (not SW-files) for higher performance – the
> guest requires more than 5 GIFs. However, the current Linux scheduler
> lacks GIF awareness, potentially scheduling >5 vCPUs to a single hart.
> This triggers VS-file allocation failure, and since no handler exists
> for this error, the QEMU guest becomes corrupted.
>
> To address this, we introduce KVM_EXIT_FAIL_ENTRY_NO_VSFILE upon
> VS-file allocation failure. This provides an opportunity for graceful
> error handling instead of corruption. For example, QEMU can handle
> this exit by rescheduling vCPUs to alternative harts when VS-file
> allocation fails on the current hart [1].

Currently, we return CSR_HSTATUS as hardware_entry_failure_reason
which is vague so it is better to return a well defined value provided via
uapi/asm/kvm.h. In general, this patch is fine but the commit description
needs to be improved along these lines.

Regards,
Anup

>
> [1] https://github.com/BillXiang/qemu/tree/riscv-vsfile-alloc/
>
> Signed-off-by: BillXiang <xiangwencheng@lanxincomputing.com>
> ---
>  arch/riscv/include/uapi/asm/kvm.h | 2 ++
>  arch/riscv/kvm/aia_imsic.c        | 2 +-
>  2 files changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/arch/riscv/include/uapi/asm/kvm.h b/arch/riscv/include/uapi/asm/kvm.h
> index 5f59fd226cc5..be29c3502fe4 100644
> --- a/arch/riscv/include/uapi/asm/kvm.h
> +++ b/arch/riscv/include/uapi/asm/kvm.h
> @@ -22,6 +22,8 @@
>  #define KVM_INTERRUPT_SET      -1U
>  #define KVM_INTERRUPT_UNSET    -2U
>
> +#define KVM_EXIT_FAIL_ENTRY_NO_VSFILE  (1ULL << 0)
> +
>  /* for KVM_GET_REGS and KVM_SET_REGS */
>  struct kvm_regs {
>  };
> diff --git a/arch/riscv/kvm/aia_imsic.c b/arch/riscv/kvm/aia_imsic.c
> index 29ef9c2133a9..69b0ab651389 100644
> --- a/arch/riscv/kvm/aia_imsic.c
> +++ b/arch/riscv/kvm/aia_imsic.c
> @@ -760,7 +760,7 @@ int kvm_riscv_vcpu_aia_imsic_update(struct kvm_vcpu *vcpu)
>                 /* For HW acceleration mode, we can't continue */
>                 if (kvm->arch.aia.mode == KVM_DEV_RISCV_AIA_MODE_HWACCEL) {
>                         run->fail_entry.hardware_entry_failure_reason =
> -                                                               CSR_HSTATUS;
> +                                                               KVM_EXIT_FAIL_ENTRY_NO_VSFILE;
>                         run->fail_entry.cpu = vcpu->cpu;
>                         run->exit_reason = KVM_EXIT_FAIL_ENTRY;
>                         return 0;
> --
> 2.46.2.windows.1

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