Re: [PATCH v2 2/2] virtio-mem: Correct default THP size for ARM64

[David Hildenbrand]

On 03.12.21 04:35, Gavin Shan wrote:
> The default block size is same as to the THP size, which is either
> retrieved from "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
> or hardcoded to 2MB. There are flaws in both mechanisms and this
> intends to fix them up.
> 
>   * When "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size" is
>     used to getting the THP size, 32MB and 512MB are valid values
>     when we have 16KB and 64KB page size on ARM64.

Ah, right, there is 16KB as well :)

> 
>   * When the hardcoded THP size is used, 2MB, 32MB and 512MB are
>     valid values when we have 4KB, 16KB and 64KB page sizes on
>     ARM64.
> 
> Co-developed-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>
> ---
>  hw/virtio/virtio-mem.c | 32 ++++++++++++++++++++------------
>  1 file changed, 20 insertions(+), 12 deletions(-)
> 
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index ac7a40f514..8f3c95300f 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/hw/virtio/virtio-mem.c
> @@ -38,14 +38,25 @@
>   */
>  #define VIRTIO_MEM_MIN_BLOCK_SIZE ((uint32_t)(1 * MiB))
>  
> -#if defined(__x86_64__) || defined(__arm__) || defined(__aarch64__) || \
> -    defined(__powerpc64__)
> -#define VIRTIO_MEM_DEFAULT_THP_SIZE ((uint32_t)(2 * MiB))
> -#else
> -        /* fallback to 1 MiB (e.g., the THP size on s390x) */
> -#define VIRTIO_MEM_DEFAULT_THP_SIZE VIRTIO_MEM_MIN_BLOCK_SIZE
> +static uint32_t virtio_mem_default_thp_size(void)
> +{
> +    uint32_t default_thp_size = VIRTIO_MEM_MIN_BLOCK_SIZE;
> +
> +#if defined(__x86_64__) || defined(__arm__) || defined(__powerpc64__)
> +    default_thp_size = (uint32_t)(2 * MiB);
> +#elif defined(__aarch64__)
> +    if (qemu_real_host_page_size == (4 * KiB)) {

you can drop the superfluous (), also in the cases below.

> +        default_thp_size = (uint32_t)(2 * MiB);

The explicit cast shouldn't be required.

> +    } else if (qemu_real_host_page_size == (16 * KiB)) {
> +        default_thp_size = (uint32_t)(32 * MiB);
> +    } else if (qemu_real_host_page_size == (64 * KiB)) {
> +        default_thp_size = (uint32_t)(512 * MiB);
> +    }
>  #endif
>  
> +    return default_thp_size;
> +}
> +
>  /*
>   * We want to have a reasonable default block size such that
>   * 1. We avoid splitting THPs when unplugging memory, which degrades
> @@ -78,11 +89,8 @@ static uint32_t virtio_mem_thp_size(void)
>      if (g_file_get_contents(HPAGE_PMD_SIZE_PATH, &content, NULL, NULL) &&
>          !qemu_strtou64(content, &endptr, 0, &tmp) &&
>          (!endptr || *endptr == '\n')) {
> -        /*
> -         * Sanity-check the value, if it's too big (e.g., aarch64 with 64k 
> base
> -         * pages) or weird, fallback to something smaller.
> -         */
> -        if (!tmp || !is_power_of_2(tmp) || tmp > 16 * MiB) {
> +        /* Sanity-check the value and fallback to something reasonable. */
> +        if (!tmp || !is_power_of_2(tmp)) {
>              warn_report("Read unsupported THP size: %" PRIx64, tmp);
>          } else {
>              thp_size = tmp;
> @@ -90,7 +98,7 @@ static uint32_t virtio_mem_thp_size(void)
>      }
>  
>      if (!thp_size) {
> -        thp_size = VIRTIO_MEM_DEFAULT_THP_SIZE;
> +        thp_size = virtio_mem_default_thp_size();
>          warn_report("Could not detect THP size, falling back to %" PRIx64
>                      "  MiB.", thp_size / MiB);
>      }
> 

Apart from that,

Reviewed-by: David Hildenbrand <david@redhat.com>


Thanks!

-- 
Thanks,

David / dhildenb