From: Andrea Arcangeli <aarca...@redhat.com> This will allow proper alignment so NPT/EPT can take advantage of linux host backing the guest memory with hugepages. It also ensures that when KVM isn't used the first 2M of guest physical memory are backed by a large TLB. To complete it, it will also notify the kernel that this memory is important to be backed by hugepages with madvise (needed for both KVM and QEMU) so that hugepages can also be used in embedded systems without any memory waste and in the future it will allow khugepaged to prioritize the collapse of hugepages into the madvise regions.
Ideally the max hugepage size provided by the transparent hugepage support in the kernel should be exported by some sysfs file, but there is no reason to expect x86_64 host to have hugepages larger than 2M or to expect those to be supported by the kernel transparent hugepage support in the short and medium term, so we can defer the invention of a fixed kernel API until this happens, by that time we'll surely have a better clue of what's the best way to provide that information to userland and it'll be a few liner change to adapt qemu to use it so there's no hurry to do it right now. Plus the below will keep to remain optimal and there is no risk of memory waste as virtual memory is practically zero cost on 64bit archs. NOTE: if the callers of qemu_ram_alloc changes significantly we may later be required to pass a second parameter to qemu_ram_alloc that will tell it what is the first guest physical address that corresponds to the "sized" memory block being allocated. I'd defer this change for later too as it may never be needed. I verified this is more than enough to get the max benefit from the kernel side feature. cat /sys/kernel/debug/kvm/largepages 301 Signed-off-by: Andrea Arcangeli <aarca...@redhat.com> --- diff --git a/exec.c b/exec.c index b0b6056..9552366 100644 --- a/exec.c +++ b/exec.c @@ -2733,11 +2733,30 @@ ram_addr_t qemu_ram_alloc(ram_addr_t size) PROT_EXEC|PROT_READ|PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0); #else - new_block->host = qemu_vmalloc(size); +#if defined(__linux__) && defined(__x86_64__) +#define MAX_TRANSPARENT_HUGEPAGE_SIZE (2*1024*1024) + if (size >= MAX_TRANSPARENT_HUGEPAGE_SIZE) + /* + * Align on the max transparent hugepage size so that + * "(gfn ^ pfn) & (HPAGE_SIZE-1) == 0" to allow KVM to + * take advantage of hugepages with NPT/EPP or to + * ensure the first 2M of the guest physical ram will + * be mapped by the same hugetlb for QEMU (it is worth + * it even without NPT/EPT). + */ + new_block->host = qemu_memalign(MAX_TRANSPARENT_HUGEPAGE_SIZE, + size); + else +#undef MAX_TRANSPARENT_HUGEPAGE_SIZE +#endif + new_block->host = qemu_vmalloc(size); #endif #ifdef MADV_MERGEABLE madvise(new_block->host, size, MADV_MERGEABLE); #endif +#ifdef MADV_HUGEPAGE + madvise(new_block->host, size, MADV_HUGEPAGE); +#endif } new_block->offset = last_ram_offset; new_block->length = size;