[PATCH v5 0/3] RISC-V: KVM: fix vcpu vector context handling

[Anup Patel]

On Tue, Aug 4, 2026 at 3:23 AM Andy Chiu <tchiu at tenstorrent.com> wrote:
>
> This series fixes a vtype corruption encountered when running perf +
> vector workload on KVM.
>
> The root cause of the bug is that the kernel-mode vector (KMV)
> misattributes the guest's vcpu context as the user's context. To solve
> this, we need to correctly save the vcpu context when the kernel-mode
> vector is serving a guest.
>
> However, calling directly into KVM from RISC-V generic architecture code
> creates a reverse dependency, which is problematic when KVM is built as
> a module. To address this, we introduce an RCU-protected callback for
> context flushing, which KVM registers during module init.
>
> Patch 1 is a preparatory cleanup that refactors
> riscv_v_start_kernel_context().
> Patch 2 prepares get/put_cpu_vector_context() for gaurding the use of
> vector in kvm_arch_vcpu_load/put()
> Patch 3 implements the callback mechanism and fixes the context handling.
>
> Reason for this respin:
>
> put_cpu_vector_context may casue a volutary ctxswch. If we prematurely
> set RISCV_PREEMPT_V, the context restore path will rasise NEED_RESTORE
> for this kernel thread. As the result, any irq taken during the
> subsequence preempt_v would trigger a restore from the empty/stale
> context memory on the irq return path in riscv_v_context_nesting_end()
>
> The fix is summarized as below:
> --- a/arch/riscv/kernel/kernel_mode_vector.c
> +++ b/arch/riscv/kernel/kernel_mode_vector.c
> @@ -181,8 +181,8 @@ static int riscv_v_start_kernel_context(void)
>         /* Transfer the ownership of V from user to kernel, then save */
>         get_cpu_vector_context();
>         __riscv_flush_vector_context();
> -       riscv_v_start(RISCV_PREEMPT_V);
>         put_cpu_vector_context();
> +       riscv_v_start(RISCV_PREEMPT_V);
>         return 0;
>  }
>
> We will send a optimization patch that removes the costly riscv_v_is_on()
> for voluntary switch detection. A kselftest that stresses the vectorized
> user copy will also be included into that series.
>
> Patch summary:
>  - unchanged patch: 1, 2
>  - new patch: none
>  - modified patch: 3
>
> Changelog v5:
>  - add the r-b from Yong-Xuan
>  - fix a guest boot fail that hits ~1/100 in host kernel-mode vector
>  - Link to v4: https://lore.kernel.org/all/20260725001749.2579274-1-tchiu@tenstorrent.com/
>
> Changelog v4:
>  - Include a header to solve a mid-series build fail (patchwork ci)
>  - Drop preempt_v_started test in may_use_simd (Sashiko)
>  - Link to v3: https://lore.kernel.org/all/20260724165001.2317788-1-tchiu@tenstorrent.com/
>
> Changelog v3:
>  - Limit the export scope for {get,put}_cpu_vector_context()
>  - clears RISCV_V_VCPU_NEED_RESTORE flag in host restore to prevent
>    leaking
>  - consolidates guest vector restore at returning to guest to prevent
>    unnecessary save/restore between the preemptible window from
>    vcpu_load to vcpu_enter_exit
>  - Document flags added to riscv_v_flags
>  - Link to v2: https://lore.kernel.org/all/20260715051629.1169645-1-tchiu@tenstorrent.com/
>
> Changelog v2:
>  - Address issues pointed out by sashiko (2, 3)
>  - Link to v1: https://lore.kernel.org/all/20260711015835.767259-1-tchiu@tenstorrent.com/
>
>
> Andy Chiu (3):
>   riscv: vector: refactor riscv_v_start_kernel_context
>   riscv: vector: allow non-preemptible kernel-mode vector with IRQs off
>   RISC-V: KVM: fix vcpu vector context handling for kernel-mode vector
>
>  arch/riscv/include/asm/kvm_vcpu_vector.h | 24 +++++++
>  arch/riscv/include/asm/processor.h       |  8 +++
>  arch/riscv/include/asm/simd.h            | 16 +----
>  arch/riscv/include/asm/vector.h          |  5 ++
>  arch/riscv/kernel/kernel_mode_vector.c   | 88 +++++++++++++++++-------
>  arch/riscv/kvm/main.c                    |  4 ++
>  arch/riscv/kvm/vcpu.c                    | 12 ++++
>  arch/riscv/kvm/vcpu_vector.c             | 22 +++++-
>  8 files changed, 139 insertions(+), 40 deletions(-)
>
> --
> 2.43.0

LGTM. I did not see any obvious issue on QEMU virt machine.

Reviewed-by: Anup Patel <anup at brainfault.org>

Queued this patch for Linux-7.3

Thanks,
Anup