> Let's allow a minimum block size of 1 MiB in all configurations. Use
> a default block size based on the THP size, and warn if something
> smaller is configured by the user.
>
> VIRTIO_MEM only supports Linux (depends on LINUX), so we can probe the
> THP size unconditionally.
>
> For now we only support virtio-mem on x86-64 - there isn't a user-visiable
> change (x86-64 only supports 2 MiB THP on the PMD level) - the default
> was, and will be 2 MiB.
>
> If we ever have THP on the PUD level (e.g., 1 GiB THP on x86-64), we
> expect to have a trigger to explicitly opt-in for the new THP granularity.
>
> Cc: "Michael S. Tsirkin" <m...@redhat.com>
> Cc: Wei Yang <richardw.y...@linux.intel.com>
> Cc: Dr. David Alan Gilbert <dgilb...@redhat.com>
> Cc: Igor Mammedov <imamm...@redhat.com>
> Cc: Pankaj Gupta <pankaj.gupta.li...@gmail.com>
> Signed-off-by: David Hildenbrand <da...@redhat.com>
> ---
> hw/virtio/virtio-mem.c | 82 +++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 78 insertions(+), 4 deletions(-)
>
> diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c
> index 8fbec77ccc..58098686ee 100644
> --- a/hw/virtio/virtio-mem.c
> +++ b/hw/virtio/virtio-mem.c
> @@ -33,10 +33,70 @@
> #include "trace.h"
>
> /*
> - * Use QEMU_VMALLOC_ALIGN, so no THP will have to be split when unplugging
> - * memory (e.g., 2MB on x86_64).
> + * Let's not allow blocks smaller than 1 MiB, for example, to keep the
> tracking
> + * bitmap small.
> */
> -#define VIRTIO_MEM_MIN_BLOCK_SIZE ((uint32_t)QEMU_VMALLOC_ALIGN)
> +#define VIRTIO_MEM_MIN_BLOCK_SIZE ((uint32_t)(1 * MiB))
> +
> +/*
> + * We want to have a reasonable default block size such that
> + * 1. We avoid splitting THPs when unplugging memory, which degrades
> + * performance.
> + * 2. We avoid placing THPs for plugged blocks that also cover unplugged
> + * blocks.
> + *
> + * The actual THP size might differ between Linux kernels, so we try to probe
> + * it. In the future (if we ever run into issues regarding 2.), we might want
> + * to disable THP in case we fail to properly probe the THP size, or if the
> + * block size is configured smaller than the THP size.
> + */
> +static uint32_t default_block_size;
> +
> +#define HPAGE_PMD_SIZE_PATH
> "/sys/kernel/mm/transparent_hugepage/hpage_pmd_size"
> +static uint32_t virtio_mem_default_block_size(void)
> +{
> + gchar *content = NULL;
> + const char *endptr;
> + uint64_t tmp;
> +
> + if (default_block_size) {
> + return default_block_size;
> + }
> +
> + /*
> + * Try to probe the actual THP size, fallback to (sane but eventually
> + * incorrect) default sizes.
> + */
> + 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) {
> + warn_report("Detected a THP size of %" PRIx64
> + " MiB, falling back to 1 MiB.", tmp / MiB);
> + default_block_size = 1 * MiB;
Probably use macro "VIRTIO_MEM_MIN_BLOCK_SIZE"
> + } else {
> + default_block_size = tmp;
> + }
> + } else {
> +#if defined(__x86_64__) || defined(__arm__) || defined(__aarch64__) || \
> + defined(__powerpc64__)
> + default_block_size = 2 * MiB;
> +#else
> + /* fallback to 1 MiB (e.g., the THP size on s390x) */
> + default_block_size = 1 * MiB;
> +#endif
Maybe we can declare this macro near to "VIRTIO_MEM_MIN_BLOCK_SIZE
((uint32_t)(1 * MiB))"
or club into one, just a thought.
> + warn_report("Could not detect THP size, falling back to %" PRIx64
> + " MiB.", default_block_size / MiB);
> + }
> +
> + g_free(content);
> + return default_block_size;
> +}
> +
> /*
> * Size the usable region bigger than the requested size if possible. Esp.
> * Linux guests will only add (aligned) memory blocks in case they fully
> @@ -437,6 +497,15 @@ static void virtio_mem_device_realize(DeviceState *dev,
> Error **errp)
> rb = vmem->memdev->mr.ram_block;
> page_size = qemu_ram_pagesize(rb);
>
> + /*
> + * If the block size wasn't configured by the user, use a sane default.
> This
> + * allows using hugetlbfs backends with a pagesize bigger than the
> detected
> + * THP size without manual intervention/configuration.
> + */
> + if (!vmem->block_size) {
> + vmem->block_size = MAX(page_size, virtio_mem_default_block_size());
> + }
> +
> if (vmem->block_size < page_size) {
> error_setg(errp, "'%s' property has to be at least the page size
> (0x%"
> PRIx64 ")", VIRTIO_MEM_BLOCK_SIZE_PROP, page_size);
> @@ -760,6 +829,12 @@ static void virtio_mem_set_block_size(Object *obj,
> Visitor *v, const char *name,
> error_setg(errp, "'%s' property has to be a power of two", name);
> return;
> }
> +
> + if (value < virtio_mem_default_block_size()) {
> + warn_report("'%s' property is smaller than the default block size "
> + "(detected THP size) of %" PRIx64 " MiB", name,
> + virtio_mem_default_block_size() / MiB);
> + }
> vmem->block_size = value;
> }
>
> @@ -810,7 +885,6 @@ static void virtio_mem_instance_init(Object *obj)
> {
> VirtIOMEM *vmem = VIRTIO_MEM(obj);
>
> - vmem->block_size = VIRTIO_MEM_MIN_BLOCK_SIZE;
> notifier_list_init(&vmem->size_change_notifiers);
> vmem->precopy_notifier.notify = virtio_mem_precopy_notify;
>
> --
> 2.26.2
>
