Re: [PATCH v2 5/7] hw/virtio/virtio-mem: Convert VIRTIO_MEM_USABLE_EXTENT to runtime

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 7/3/25 23:39, Philippe Mathieu-Daudé wrote:
> Use qemu_arch_available() to check at runtime if a target
> architecture is built in.
> 
> Consider the maximum extent size of any architecture built in.
> 
> Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> ---
>   hw/virtio/virtio-mem.c | 28 ++++++++++++++++++++--------
>   1 file changed, 20 insertions(+), 8 deletions(-)


> @@ -170,13 +171,24 @@ static bool virtio_mem_has_shared_zeropage(RAMBlock *rb)
>    * necessary (as the section size can change). But it's more likely that the
>    * section size will rather get smaller and not bigger over time.
>    */
> -#if defined(TARGET_X86_64) || defined(TARGET_I386) || defined(TARGET_S390X)
> -#define VIRTIO_MEM_USABLE_EXTENT (2 * (128 * MiB))
> -#elif defined(TARGET_ARM)
> -#define VIRTIO_MEM_USABLE_EXTENT (2 * (512 * MiB))
> -#else
> -#error VIRTIO_MEM_USABLE_EXTENT not defined
> -#endif
> +static uint64_t virtio_mem_usable_extent_size(void)
> +{
> +    uint64_t size = 0;
> +
> +    assert(qemu_arch_available(QEMU_ARCH_ARM | QEMU_ARCH_I386 | QEMU_ARCH_S390X));

I'm not sure this assertion is doing what I thought it'd do.

For example, building with --target-list=aarch64-softmmu,riscv32-softmmu,
this device is now linked in. However, riscv32 machines won't be able
to plug it until they allow TYPE_VIRTIO_MD_PCI in some of their
HotplugHandlerClass handlers. Still I'd like to catch this case here
to avoid bad surprises.

> +    /*
> +     * FIXME: We should use the maximum of instantiated vCPUs ARCH, but
> +     *        for now it is easier to take the maximum of any ARCH built in.
> +     */
> +    if (qemu_arch_available(QEMU_ARCH_I386 | QEMU_ARCH_S390X)) {
> +        size = MAX(size, 2 * 128 * MiB);
> +    }
> +    if (qemu_arch_available(QEMU_ARCH_ARM)) {
> +        size = MAX(size, 2 * 512 * MiB);
> +    }
> +
> +    return size;
> +}