The RISC-V Privileged specificaiton says the following: "WFI is also
required to resume execution for locally enabled interrupts pending
at any privilege level, regardless of the global interrupt enable at
each privilege level."

Based on the above, if there is pending VS-timer interrupt when the
host (aka HS-mode) executes WFI then such a WFI will simply become NOP
and not do anything. This result in QEMU RISC-V consuming a lot of CPU
time on the x86 machine where it is running. The PATCH1 solves this
issue by adding appropriate cleanup in KVM RISC-V timer virtualization.

As a result PATCH1, race conditions in updating HGEI[E|P] CSRs when a
VCPU is moved from one host CPU to another are being observed on QEMU
so the PATCH2 tries to minimize the chances of these race conditions.

Changes since v1:
 - Added more details about race condition in PATCH2 commit description.

Anup Patel (2):
  RISC-V: KVM: Disable vstimecmp before exiting to user-space
  RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization

 arch/riscv/include/asm/kvm_aia.h |  4 ++-
 arch/riscv/kvm/aia.c             | 51 +++++---------------------------
 arch/riscv/kvm/aia_imsic.c       | 45 ++++++++++++++++++++++++++++
 arch/riscv/kvm/vcpu.c            |  2 --
 arch/riscv/kvm/vcpu_timer.c      | 16 ++++++++++
 5 files changed, 71 insertions(+), 47 deletions(-)

-- 
2.43.0

On Mon, Jul 7, 2025 at 9:23 AM Anup Patel <apatel@ventanamicro.com> wrote:
>
> The RISC-V Privileged specificaiton says the following: "WFI is also
> required to resume execution for locally enabled interrupts pending
> at any privilege level, regardless of the global interrupt enable at
> each privilege level."
>
> Based on the above, if there is pending VS-timer interrupt when the
> host (aka HS-mode) executes WFI then such a WFI will simply become NOP
> and not do anything. This result in QEMU RISC-V consuming a lot of CPU
> time on the x86 machine where it is running. The PATCH1 solves this
> issue by adding appropriate cleanup in KVM RISC-V timer virtualization.
>
> As a result PATCH1, race conditions in updating HGEI[E|P] CSRs when a
> VCPU is moved from one host CPU to another are being observed on QEMU
> so the PATCH2 tries to minimize the chances of these race conditions.
>
> Changes since v1:
>  - Added more details about race condition in PATCH2 commit description.
>
> Anup Patel (2):
>   RISC-V: KVM: Disable vstimecmp before exiting to user-space
>   RISC-V: KVM: Move HGEI[E|P] CSR access to IMSIC virtualization
>
>  arch/riscv/include/asm/kvm_aia.h |  4 ++-
>  arch/riscv/kvm/aia.c             | 51 +++++---------------------------
>  arch/riscv/kvm/aia_imsic.c       | 45 ++++++++++++++++++++++++++++
>  arch/riscv/kvm/vcpu.c            |  2 --
>  arch/riscv/kvm/vcpu_timer.c      | 16 ++++++++++
>  5 files changed, 71 insertions(+), 47 deletions(-)
>

Queued this series as fixes for Linux-6.16

I have taken care of the comment on PATCH2 at the time of queuing.

Thanks,
Anup