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; + } 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 + 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