Re: [PATCH v2 4/7] target/i386/mshv: hv_vp_register_page setup for the vcpu

[Magnus Kulke <magnuskulke@linux.microsoft.com>]

On Tue, May 05, 2026 at 09:50:25PM +0300, Doru Blânzeanu wrote:
> When the vcpu is created, call mmap to configure access to the register page.
> In case the call to mmap fails, we log an error and continue with the
> previous logic (using hypercalls).
> 
> Update CPUArchState to store a pointer to the mmapped hv_vp_register_page.
> 
> Signed-off-by: Doru Blânzeanu <dblanzeanu@linux.microsoft.com>
> ---
>  target/i386/cpu.h           |  5 +++++
>  target/i386/mshv/mshv-cpu.c | 22 ++++++++++++++++++++++
>  2 files changed, 27 insertions(+)
> 
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 16de67e546..fd4c3712b1 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -2019,6 +2019,11 @@ typedef struct CPUArchState {
>      uint64_t msr_bndcfgs;
>      uint64_t efer;
>  
> +#ifdef CONFIG_MSHV
> +    /* Shared register page */
> +    struct hv_vp_register_page *regs_page;
> +#endif

since this is declared before this marker

    /* Fields up to this point are cleared by a CPU reset */
    struct {} end_reset_fields;

the pointer will be zeroed out on init, best move it after the
emu_mmio_buf field.

> +
>      /* Beginning of state preserved by INIT (dummy marker).  */
>      struct {} start_init_save;
>  
> diff --git a/target/i386/mshv/mshv-cpu.c b/target/i386/mshv/mshv-cpu.c
> index 9defd05db6..3a3c269c33 100644
> --- a/target/i386/mshv/mshv-cpu.c
> +++ b/target/i386/mshv/mshv-cpu.c
> @@ -1587,6 +1587,7 @@ void mshv_arch_init_vcpu(CPUState *cpu)
>      CPUX86State *env = &x86_cpu->env;
>      AccelCPUState *state = cpu->accel;
>      size_t page = HV_HYP_PAGE_SIZE;
> +    void *regs_page;
>      void *mem = qemu_memalign(page, 2 * page);
>  
>      /* sanity check, to make sure we don't overflow the page */
> @@ -1595,6 +1596,22 @@ void mshv_arch_init_vcpu(CPUState *cpu)
>                        + sizeof(hv_input_get_vp_registers)
>                        > HV_HYP_PAGE_SIZE));
>  
> +
> +    /* mmap the registers page */
> +    regs_page = mmap(NULL, page, PROT_READ | PROT_WRITE,
> +                    MAP_SHARED, mshv_vcpufd(cpu),
> +                    MSHV_VP_MMAP_OFFSET_REGISTERS * page);
> +    if (regs_page == MAP_FAILED) {
> +        /*
> +         * Error is not fatal, but we won't be able to use the
> +         * fast path for register access
> +         */
> +        error_report("register page mmap failed: %s", strerror(errno));
> +        env->regs_page = NULL;
> +    } else {
> +        env->regs_page = (struct hv_vp_register_page *) regs_page;
> +    }
> +
>      state->hvcall_args.base = mem;
>      state->hvcall_args.input_page = mem;
>      state->hvcall_args.output_page = (uint8_t *)mem + page;
> @@ -1608,6 +1625,11 @@ void mshv_arch_destroy_vcpu(CPUState *cpu)
>      CPUX86State *env = &x86_cpu->env;
>      AccelCPUState *state = cpu->accel;
>  
> +    /* Unmap the register page */
> +    if (env->regs_page) {
> +        munmap(env->regs_page, HV_HYP_PAGE_SIZE);
> +        env->regs_page = NULL;
> +    }
>      g_free(state->hvcall_args.base);
>      state->hvcall_args = (MshvHvCallArgs){0};
>      g_clear_pointer(&env->emu_mmio_buf, g_free);
> -- 
> 2.53.0