Re: [PATCH] target/i386/whpx: handle processor reset

> On 18. Aug 2026, at 19:58, Doug Cook (WINDOWS) <[email protected]> wrote:
> 
> WHPX is not properly handling processor reset:
> 
> - Does not reset partition, so several registers (e.g. Hypercall,
>  GuestOsId) carry values forward as "Locked" into the next session.
> - Does not clear hypervisor-managed TSC deadline (live-lock).
> - Does not re-arm the wait-for-SIPI trigger, so all CPUs start running
>  immediately after reset.
> 
> Some firmware may tolerate this, but OVMF does not. Typical result is
> that boot hangs or ends in triple-fault, e.g.
> 
>  qemu-system-x86_64: WHPX: Unexpected VP exit code 4
> 
> After this patch, I am able to boot to Windows desktop, reset, and boot
> back to Windows desktop. Tried many times in sequence with no problems.
> Used several different reset mechanisms (OS shutdown menu, external
> "reset", watchdog reset) with no problems.
> 
> Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/2402
> Signed-off-by: Doug Cook <[email protected]>

Hi,

Previous version of this concept at: https://patchew.org/QEMU/[email protected]/ (with it intertwined in a common path for Arm support)

Using WHvResetPartition as a workaround for some state not being fully synchronised isn’t
ideal though...

> ---
> target/i386/whpx/whpx-all.c | 29 +++++++++++++++++++++++++++++
> 1 file changed, 29 insertions(+)
> 
> diff --git a/target/i386/whpx/whpx-all.c b/target/i386/whpx/whpx-all.c
> index 634d542821..54f9b50536 100644
> --- a/target/i386/whpx/whpx-all.c
> +++ b/target/i386/whpx/whpx-all.c
> @@ -528,6 +528,35 @@ void whpx_set_registers(CPUState *cpu, WHPXStateLevel level)
> 
>     assert(cpu_is_stopped(cpu) || qemu_cpu_is_self(cpu));
> 
> +    /* Assumes that cpu_synchronize_all_post_reset() runs cpu 0 first. */
> +    if (level == WHPX_LEVEL_RESET_STATE && cpu->cpu_index == 0) {
> +        hr = whp_dispatch.WHvResetPartition(whpx->partition);
> +        if (FAILED(hr)) {
> +            error_report("WHPX: Failed to reset partition, hr=%08lx", hr);
> +        }
> +    }
> +
> +    /* Reset hypervisor-owned TSC deadline. */
> +    if (level == WHPX_LEVEL_RESET_STATE) {
> +        WHV_REGISTER_VALUE tsc_deadline = {};
> +
> +        whpx_set_reg(cpu, WHvX64RegisterTscDeadline, tsc_deadline);
> +    }
> +
> +    /*
> +     * Re-arm partition's wait-for-SIPI state.
> +     * whpx_vcpu_run() ignores cpu->halted, depends on SIPI for this.
> +     * This must follow WHvResetPartition.
> +     */
> +    if (level == WHPX_LEVEL_RESET_STATE && whpx_irqchip_in_kernel()) {
> +        WHV_REGISTER_VALUE activity = {};
> +
> +        whpx_get_reg(cpu, WHvRegisterInternalActivityState, &activity);
> +        activity.InternalActivity.StartupSuspend = !cpu_is_bsp(x86_cpu);
> +        activity.InternalActivity.HaltSuspend = 0;
> +        whpx_set_reg(cpu, WHvRegisterInternalActivityState, activity);
> +    }
> +
>     /*
>      * Following MSRs have side effects on the guest or are too heavy for
>      * runtime. Limit them to full state update.
> -- 
> 2.55.0.vfs.0.3
>