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SubjectRe: [PATCH 04/15] KVM: selftests: Force stronger HVA alignment (1gb) for hugepages
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On 2021/2/11 7:06, Sean Christopherson wrote:
> Align the HVA for hugepage memslots to 1gb, as opposed to incorrectly
> assuming all architectures' hugepages are 512*page_size.
>
> For x86, multiplying by 512 is correct, but only for 2mb pages, e.g.
> systems that support 1gb pages will never be able to use them for mapping
> guest memory, and thus those flows will not be exercised.
>
> For arm64, powerpc, and s390 (and mips?), hardcoding the multiplier to
> 512 is either flat out wrong, or at best correct only in certain
> configurations.
>
> Hardcoding the _alignment_ to 1gb is a compromise between correctness and
> simplicity. Due to the myriad flavors of hugepages across architectures,
> attempting to enumerate the exact hugepage size is difficult, and likely
> requires probing the kernel.
>
> But, there is no need for precision since a stronger alignment will not
> prevent creating a smaller hugepage. For all but the most extreme cases,
> e.g. arm64's 16gb contiguous PMDs, aligning to 1gb is sufficient to allow
> KVM to back the guest with hugepages.
I have implemented a helper get_backing_src_pagesz() to get granularity
of different
backing src types (anonymous/thp/hugetlb) which is suitable for
different architectures.
See:
https://lore.kernel.org/lkml/20210225055940.18748-6-wangyanan55@huawei.com/
if it looks fine for you, maybe we can use the accurate page sizes for
GPA/HVA alignment:).

Thanks,

Yanan
> Add the new alignment in kvm_util.h so that it can be used by callers of
> vm_userspace_mem_region_add(), e.g. to also ensure GPAs are aligned.
>
> Cc: Ben Gardon <bgardon@google.com>
> Cc: Yanan Wang <wangyanan55@huawei.com>
> Cc: Andrew Jones <drjones@redhat.com>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Aaron Lewis <aaronlewis@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> tools/testing/selftests/kvm/include/kvm_util.h | 13 +++++++++++++
> tools/testing/selftests/kvm/lib/kvm_util.c | 4 +---
> 2 files changed, 14 insertions(+), 3 deletions(-)
>
> diff --git a/tools/testing/selftests/kvm/include/kvm_util.h b/tools/testing/selftests/kvm/include/kvm_util.h
> index 4b5d2362a68a..a7dbdf46aa51 100644
> --- a/tools/testing/selftests/kvm/include/kvm_util.h
> +++ b/tools/testing/selftests/kvm/include/kvm_util.h
> @@ -68,6 +68,19 @@ enum vm_guest_mode {
> #define MIN_PAGE_SIZE (1U << MIN_PAGE_SHIFT)
> #define PTES_PER_MIN_PAGE ptes_per_page(MIN_PAGE_SIZE)
>
> +/*
> + * KVM_UTIL_HUGEPAGE_ALIGNMENT is selftest's required alignment for both host
> + * and guest addresses when backing guest memory with hugepages. This is not
> + * the exact size of hugepages, rather it's a size that should allow backing
> + * the guest with hugepages on all architectures. Precisely tracking the exact
> + * sizes across all architectures is more pain than gain, e.g. x86 supports 2mb
> + * and 1gb hugepages, arm64 supports 2mb and 1gb hugepages when using 4kb pages
> + * and 512mb hugepages when using 64kb pages (ignoring contiguous TLB entries),
> + * powerpc radix supports 1gb hugepages when using 64kb pages, s390 supports 1mb
> + * hugepages, and so on and so forth.
> + */
> +#define KVM_UTIL_HUGEPAGE_ALIGNMENT (1ULL << 30)
> +
> #define vm_guest_mode_string(m) vm_guest_mode_string[m]
> extern const char * const vm_guest_mode_string[];
>
> diff --git a/tools/testing/selftests/kvm/lib/kvm_util.c b/tools/testing/selftests/kvm/lib/kvm_util.c
> index deaeb47b5a6d..2e497fbab6ae 100644
> --- a/tools/testing/selftests/kvm/lib/kvm_util.c
> +++ b/tools/testing/selftests/kvm/lib/kvm_util.c
> @@ -18,7 +18,6 @@
> #include <unistd.h>
> #include <linux/kernel.h>
>
> -#define KVM_UTIL_PGS_PER_HUGEPG 512
> #define KVM_UTIL_MIN_PFN 2
>
> /*
> @@ -670,7 +669,6 @@ void vm_userspace_mem_region_add(struct kvm_vm *vm,
> {
> int ret;
> struct userspace_mem_region *region;
> - size_t huge_page_size = KVM_UTIL_PGS_PER_HUGEPG * vm->page_size;
> size_t alignment;
>
> TEST_ASSERT(vm_adjust_num_guest_pages(vm->mode, npages) == npages,
> @@ -733,7 +731,7 @@ void vm_userspace_mem_region_add(struct kvm_vm *vm,
>
> if (src_type == VM_MEM_SRC_ANONYMOUS_THP ||
> src_type == VM_MEM_SRC_ANONYMOUS_HUGETLB)
> - alignment = max(huge_page_size, alignment);
> + alignment = max((size_t)KVM_UTIL_HUGEPAGE_ALIGNMENT, alignment);
> else
> ASSERT_EQ(src_type, VM_MEM_SRC_ANONYMOUS);
>

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